Should I buy a pregnancy test?

Ok, so on Saturday, 11/13 I had this semi- with my bf, we're both 21. I say semi- because it was quite painful and I asked him to stop. I have been on Yaz since May and have never missed a pill. I also have Crohn's disease which can cause diarrhea, so whenever I've taken my pill and had an attack immediately after, I have taken another pill just in case to make sure I absorb it. My bf was also wearing a condom, and he didn't , but may have pre-d but he wasn't sure. He even filled the condom with water and nothing squirted out so we umed it was intact. On Saturday, 11/20, I got my period like clockwork, one week after the encounter. I thought it was a heavy to normal flow, especially on the first day and second day I had it, it started to lessen by the third day, and by the 4th day, Tuesday, it was very light, very little on the pad. Same on Wednesday, almost nonexistent, and just sort of stopped. My question is that I usually bleed for almost a week, and I'm very scared this is a sign of pregnancy, as it just sort of stopped after 3-4 days, (I'm counting Saturday as my first day, since that's when I started to bleed). I'm so upset and nervous, could this short period just be stress? I was really upset after the and stressed about school, (I'm in college). Should I call my doctor? Should I take the test? Thank you in advance and reading my long story.