stomach hurts

a while ago i sorta stopped eating because i just grew tired of eating (too much food in the house) but i would still eat daily because i knew i needed to but not as much as before. that lasted for about 2 weeks then i started eating alot and my stomach began to feel really bloated this was the first, it felt as if the top part of my stomach was going to burst any minute. i became constipated for a few days. i then had unprotected sex with my boyfriend and i thought i was pregnant but i had my period, but i'm still not sure. a few of these symptoms came after we had sex. i've seen on the internet where it could be ibs but i'm not sure. can anyone help.