Suddenly I have problems with anal sex when I bottom (why? and looking for advice)

I have a very embarrassing question about anal sex. I was a virgin until two month ago. First when this guy (not my boyfriend) and I started having sex everything was fine. He also has some experience and knows what to do. But for the past 3 weeks I have diarrhea ever morning after I had anal sex the night before. It is really bad now and sometimes I spent more than 20 minutes on the toilet until everything ‘calms down’ :-( also sometimes I feel a very sharp pain deep guts. I have already asked him to pull out before he finishes so that I don’t get a “semen enema” every time but so far nothing changed.Why is it happening now? Do you have any advice?At the beginning everything was fine and then the problems started… I did some research online but it did not really help. I know I should go to a doctor but that is not possible until I turn 18 because of our insurance. Let me explain, because I am not sure how it works in other countries. Basically my parents will get the doctor’s bill and then my parents forward it to our insurance and they will pay for it. So when I got to the doctor my parents will ask for the reason and I cannot tell them.Sorry about my English I am not a native speaker. I’m just really paranoid about my family finding out so I prefer it not to use my mother tongue.