For the past 2 weeks my 3 1/2 yo. girl has been urinating in her dress without telling anyone in school. Before that she used the bathroom in school. She has been potty trained for more than a year, but at home she goes to the bathroom and she tells me she wants to pee. Is this the starting stage of a disease?
If the problem is only at school, I don't think it's a disease (=illness), but possibly an indication of a dis-ease (=feeling uncomfortable).
If she had a disease like a urinating tract infection, she'd show symptoms at home, too. So unless you have noticed abnormal behaviour like frequent urination or discomfort, I wouldn't worry.
For sudden relapses related to a certain place like school, try to find out if something happened.
This may be as simple as the teachers stopping to remind their pupils to go to the bathroom (and your child may simply forget in the general excitement or for fear of missing something) or her misunderstanding a rule or instruction like "don't interrupt the lesson" - and then staying put even though she feels the need to go.
Or she had a negative experience related to bathroom use at school - seemingly minor events like another child pushing her can be pretty frightening. A more malicious scenario could be others keeping her from using the bathroom because they want her to embarrass herself, but this kind of bullying is more unlikely.
Talk to your child.
Try to find out whether a scenario like above happened or what else may make her uncomfortable. No child wants to wet herself and her not telling indicates that she tries to hide it. Don't scold or punish her when she comes home wet, but show her that you listen and want to help. Then, together find a solution to whatever is bothering her.
I'd be very careful how you ask your child about this. Perhaps you could ask the teacher if there have been any changes at school. Was the teacher absent? Was your daughter punished? Was your daughter bullied -- or is it a possibility?
Please know that any sudden change along these lines is a concern and you are absolutely right to want to understand.
Talk to the doctor ahead of a visit and then s/he can see if there is a medical reason for the change.