Doubting who's baby this is! Horrible anxiety during pregnancy, scared.. Help?

I'm 20. Please read carefully.

I'm about 21 or 22 weeks pregnant now.

Anyways, I've been having this problem for the past week, and all of a sudden my anxiety is out of control!! It just came out of nowhere.

Well, I'm due around the 6th of August.

In October I had sex twice with an old fling WITH condoms I'm not sure of the dates. Me and ex John also hooked up as well but we never use protection (he never pulls out) it all happened BEFORE the 24th of October.

I DID get my period on November 2nd, it wasn't spotting, it wasn't different at all, it was my normal 7-8 day period!

I ended up hooking back up with John on November 16th, unprotected and he didn't pull out. After November I never got a period again. December is when all the morning sickness happened and lasted until about February, along with other symptoms.

I got an ultrasound on February 13th which said the fetus was 13 weeks, and that proves it's Johns, the doctor said according to my last period I'm actually 15 weeks which makes sense because doctors always add 2 weeks to pregnancy.

I'm worrying myself to death, I can't eat, I sleep, everything was fine and I had absolutely no doubt in the world about who's baby this was, and all of sudden I'm thinking it's someone who I hooked up with the month before.. even after a period and everything!! :( please tell me I'm just stressing myself out for no reason..