pregnancy question ! !

Okay me && my boyfriend have been having unprotected sex for 2 months straight, almost every night. he cummed every time, he did not pull out or anything. i am not on any kind of birth control. I have had 2 irregular periods since we have been haveing sex, usally my periods are 5 days long or longer, and are very heavy periods they normaly happen at the end of each month. well my first irregular period was 3 days long and very light but had maybe one huge blood clot during the 3 days.  then my second one was 1 day but i had another day of spotting. both were in the beginning of the months.ive been having really bad cramps and i am usaually very thin with a flat stomich and now my stomich is getting fatter and fatter and i dont know how. now, i took 2 pregnancy test from the dollar store about a week apart. they both said negative. What are the possibility that i am pregnant ? what would be the reasons ive been testing negative if im prego ?