Possible Pregnancy Scare :o

So I'm 20 years old and recently started having safe sex with my boyfriend. We use a condom every time and I'm on birth control pills. We last had sex about 3 weeks ago. Within a week from that I was having a lot of nausea, so I took a pregnancy test to be on the safe side. It was negative so I went to the doctor. They gave me a diagnosis of h-pylori and told me to see a gastroenterologist. I also had a low potassium level so I was put on a supplement. It's been a little over a week and I'm supposed to get my period this week (<a href="https://play.google.com/store/apps/details?id=com.glow.android.eve">Eve</a> says today). I've been having horrible cramps since yesterday afternoon. This morning I went to the bathroom and I got a sharp pain in my uterus. It was so bad I almost passed out. When I wiped myself (sorry if that's tmi), there was a little bit of blood, so I thought maybe I got my period, but I've barely bled at all since (and I usually have heavy periods). The cramps are still so bad that I end up doubling over when I try to walk. Does anyone know what's happening to me? Should I take another test?