Period question

Hi I haven't had my period in the last 4 months (doctor said its because of my weight loss) so the boyfriend & I have been having sex unprotected. 
Friday I was shopping (stress free) I got this sharpe pain threw my vagina/ stomach area. Went to the loo because I felt wet. I wasn't bleeding. And hour later I started to bleed but it only lasted about 15 minutes and it was so light. Now I'm cramping with no bleeding. My nips feel funny. Back aches, head ache & all I want to do is eat & cry what could it be?