I didn't use a condom for 1 push

I pentrated into a girl once without condom. But before we were using a condom so Im afraid fluids still may have gotten into her just off of one penetration without a condom. what should I do?

it might tell her to see a doctor

Your pre-cum and her natural lubrication both can transmit STDs, and your pre-cum may contain sperm, so even just one push can be enough to cause a pregnancy or a disease if you didn't have a condom on. If you don't trust each other that you're clean from diseases, both of you sould get yourselves checked by a doctor. She may also see one (or take a test) to see if she's pregnant. It's rare but not impossible.