What Causes Irregular Periods With Cramps?

I am a 19 year old female I had what I guess you could call "sex" around March 14th.. I still have not received my period and have absolutely no signs of pregnancy, I also took 2 pregnancy tests 2-3 weeks (took them a week apart) ago both coming back negative! Within this last week I started having cramps and bleeding but not how I normally do during my period looks like actual blood, while sometimes it looks exactly like my period with some clots like I would normally receive with my period! I am irregular most of the time, however not to this extent! What is going on?