Potty Anxiety ? Help!

Until I take my daughter to the pediatrician I wanted to check here to see if any other parent(s) has been going through same/similar issue.

My daughter just turned 3 in May and has been potty trained for three months. She overall does well going to the potty both pee and poop, but sometimes will have the urge to pee, and in the middle of what looks to be a tantrum, won’t sit on the potty when we put her there. She will thrash her legs scream and cry and won’t sit still and we are left trying to figure out how to get her to calm down enough to get her to relieve herself. Again, this doesn’t happen often but when it does we have no clue what to do.