I'm 19 years old and sexually active. I always ensure that if i'm having sex it is protected, and i have only had sex with one guy. MY period comes every 28 days for 5 days. In January it came 7 days early which is quite unusual and it is now March and it hasn't returned. 4 weeks after the last time i had sex (last week) i took 2 pregnancy tests because my period was now 2 weeks late and both showed up negative. I have been experiencing unusual stomach pains that sometimes intensify and i also experienced an intense pain in my abdomen and my back that i had felt 2 years before i started having sex. I had sex in early january and twice in the first 2 weeks of february, we used a condom for each occasion and even checked for holes after wards. The stomach pains happen all, sometimes close to my chest and sometimes closer to my abdomen.

What to do? What could be the problem ??