Daughter is recently afraid of peeing in potty

My wife and I need help with our 3 year old daughter.  She recently has stopped going to the potty at school and a few days later at home.  She used to go on her own very well.  She said she is scared of the potty.  Something has seemed to scare her from the potty at school.  We are now trying to offer rewards like stickers or small toys if she is successful or tries to go.  Unfortunately we tried to push her and tell her to go and that made it worse.  She cries very badly about it.  After she goes on herself it is very difficult to deal with her because she does not want new clothes on afterwards.  She refuses (very strong willed) to put on new clothes and is a major problem at the nursery school because of this.  Our new strategy is to not tell her about the potty at all or push her at all, we will just reward success.  We will happily clean up messes, give her baths, and try our best to re-dress her.  This means some hard times missing work to pick her up from school since she may refuse to wear clothes, have temper tantrums, etc.   The pediatrician did not find any urinary problems, etc. but it seems something scared her and she will not tell us anything in particular.

Please advise!