TL:DR: You have a choice: ease up on your child, or push for an evaluation. (Edited to add: one week of picky eating does not qualify your child for receiving further evaluation; this is discussed as a general problem.)
There are a lot of reliable resources on the internet providing guidance in feeding. The focus of many are on the prevention of obesity, but they address picky eaters as well. One conceptual framework pops up in both, something called the "division of responsibility in feeding". It helps everyone: anxious parents of overeaters, anxious parents of picky eaters, and health care providers: doctors, nutritionists, etc.
In Child Care Provider Training and a Supportive Feeding Environment in Child Care Settings in 4 States, 2003, the issue of eating behavior in daycare/preschool is addressed because of the amount of time impoverished kids spend in such facilities and the issue of obesity in the poor. It is recommended that the Slatter Division of Responsibily be implemented because of its success in teaching children to eat "properly", that is, to have a healthy relationship with food.
Satter's widely accepted concept postulates a distribution of feeding and eating roles between child and adult. Adult caregivers are responsible for selecting, preparing, and offering healthful, appropriate foods and determining when and where food is served. Children are responsible for how much of these foods they eat and whether they eat at all.
A healthy relationship with food (eating when hungry; eating healthful foods) starts, according to Slatter,
Division of Responsibility is about people doing their jobs and the other people trusting them to do their jobs. When children are infants, they have all the responsibility knowing when to nurse, and they trust the adults who care for them to respond to their needs. As they develop and become more regular in their eating patterns, their parents take on more responsibility for deciding what, when, and where their children will eat — and they trust their children to intuitively know which foods to say yes to and how much to eat. [emphasis mine]
The division of responsibility in feeding applies at every stage in your child’s growing-up years, from infancy through the early years through adolescence. Throughout, parents maintain the structure of family meals and sit-down snacks throughout the growing-up years.
The method is simple: family meals, good parental modelling of healthy meals, regular snack times, no between meal/snack feeding except water, and not fussing over what the child does and doesn't eat (trust your child to eat; that's their responsibility. Fuss and you create an unhealthy environment around food.)
Tell her, “you don’t have to eat,” and show her you mean it. Don’t pressure her in any way to eat. Don’t talk about her food likes and dislikes.
Make meals a pleasure and a privilege. Tell her, “you do have to behave at mealtime. Say ‘yes please’ and ‘no thanks.’ No whining, asking for special food, or saying ‘yuk.’” Show her you mean it.
The Ellen Slatter Institute has a page for parents of picky eaters;
Pressure on children’s eating always backfires...
What does pressure on eating look like?
Pressure can seem positive: Praising, reminding, bribing, rewarding, applauding, playing games, talking about nutrition, giving stickers, going on and on about how great the food is, making special food, serving vegetables first, making food fun (to get the child to eat, not just to have fun).
Pressure can be negative: Restricting amounts or types of food, coaxing, punishing, shaming, criticizing, begging, withholding dessert, treats, or fun activities, physically forcing, threatening.
Pressure can seem like good parenting: Insisting on “no thank you” bites, encouraging or reminding the child to eat, taste, smell or lick, making her eat her vegetables, warning her that she will be hungry, making special food, keeping after her to use her silverware or napkin, hiding vegetables in other foods, letting her eat whenever she wants to between meals.
Pressure can be hard to detect: Ask yourself why you are doing something with feeding. Is it to get your child to eat more, less or different food than he does on his own? If so, it is pressure.
Having written all this, I don't agree with all of Slatter's methods or even principles.
I can state that picky eaters are so common that parental concerns are usually blown off by the doctor (see Picky eating during childhood: A longitudinal study to age 11-years and Trajectories of picky eating during childhood: A general population study.), which is why I am amazed - really amazed - that your doctor, who found your child healthy, would recommend force feeding. But this has been a subject of more research.
For example, children with autism are often selective eaters, so much so that they were eliminated in one study, Psychological and Psychosocial Impairment in Preschoolers With Selective Eating.
The study reports that picky eaters sometimes merit a more involved approach from physicians, that they should look for anxiety disorders or ADHD in picky eaters.
Both moderate and severe levels of SE [Selective Eating] were associated with psychopathological symptoms (anxiety, depression, attention-deficit/hyperactivity disorder) both concurrently and prospectively. However, the severity of psychopathological symptoms worsened as SE became more severe. Impairment in family functioning was reported at both levels of SE, as was sensory sensitivity in domains outside of food and the experience of food aversion.
Please note that this concern (of selective eating) is not addressed from a nutritional standpoint but a psychological one.
The pickiest eater I ever had as a patient was one who ate from exactly three foods and only three foods: plain cheese pizza, broiled chicken, and pretzels (He continued this way until into his marriage.) He had/has extreme tactile defensiveness issues (could not tolerate long pants for example; would ski and got married in shorts.) His parents refused testing for Autism Spectrum Disorder but allowed it for ADHD, which was present.
I suspect he was/is on the high-functioning end of ASD, but physically (on that diet) he grew into a strong, healthy, handsome and slightly overweight adult, married with children. I am amazed, but there it is. For many reasons not elaborated on here, but including that they didn't make a terrible fuss about what their child ate, loving him for exactly who and what he was, I credit his parents to be among the best I've ever known.
