Will a doctor tell a parent if I had sexual activty

In most cases it depends on the situation. They'll try to respect your privacy but if you are a minor the doctor needs your parents permission for some tests/procedures. In these cases he may have to disclose that information.

What is the nature of your concern?

if you get examined and everything is ok and you request that they dont say anything then they cant say anything by law, however if something is wrong or they want to check just to be sure and you are not 18 then they would need to get permission but it is still possible that you can talk to you doctor about not mentioning that you are sexualy active and they usually have ways of asking parents without mentioning that part