I've Been With My Boyfriend For About Nine Months, And

I've been with my boyfriend for about nine months, and starting in about December, soon after we became sexually active, I missed that
month's period and January's. I lost my virginity to him and I'd also never missed a period before in my life. I took two pregnancy tests and they both came out negative, so I tried to relax. In February I had my period and I thought everything was fine again. Then I missed March, April, and I didn't have one this month either. In early March we had a broken condom mishap so once again, I took a pregnancy test and another one about a month later. Negative both times. I have been progressively gaining weight since we've been together because my eating habits have gotten bad, and I read it's normal to miss a month from weight gain, but after missing several months when I never previously had a problem I'm worried it could be something worse. I'm 19 and absolutely terrified of the possibility of going to a gynecologist, and I don't know what's going on with my body.