Agh. Supplementing is such bad advice (regardless of f the advice is to give additional breast milk or formula), it really upsets me that pediatricians (who should know better!) still tell people to do this, especially in the case of slow weight gain.
Advice concerning slow weight gain (or weight loss) is very tricky, as there are so many different factors to consider, and rarely just one cause. Slow weight gain may be a sign of underlying problems in the child (but the most common cause is still problems with the breastfeeding technique). To routinely advise supplementation without finding the underlying cause of the slow weight gain or weight loss can, in the worst case, conceal sickness in the child.
It is important to understand that more than 95% of the newborns lose weight after birth because of natural fluid loss, combined with the naturally low volume of colostrum the baby ingests the first few days. Around 12% of babies lose 10% or more in weight, ans only 5% increase in weight the first three days [January Riordan, Breastfeeding and human lactation, 2010]. Hence, it's completely normal for the child to lose weight after birth.
One should also keep in mind that weight is not the only measure of growth. Measuring height and head circumference may be just as important (if not more). Just because a child is gaining weight does not mean she is growing, and rapid weight gain is not always something worth striving for.
In Sweden, recommendations to breastfeeding counselors is to not advice to start supplementing unless the child has lost more than 10% of its birth weight. Hence it's perfectly okay for a child to lose up to 10% after birth, without the need for supplementation.
At a 5-7% weight loss the pediatrician should focus on supporting the mother and the baby with their breastfeeding technique, make sure the child has as good a latch as possible, helping to reduce and ease any redress or pain, and talking about how the mother can increase her milk production (basic anatomy, how milk production goes to, etc.). Instructing the mother to supplement is known to be connected with low self-esteem of the mother in regard to breastfeeding and can cause problems with milk production.
Another problem is that your baby will get used to drinking out of a bottle, which can lead to nipple confusion and make breastfeeding even harder for the pair of you. I would stop doing this immediately, and focus on getting the breastfeeding to work instead. Your baby will not starve, you need to trust her on this.
Contact a lactation consultant ASAP to get help with your breastfeeding. Good luck!