My insides burn what's wrong with me?

I have been having problems for the past month. First of all it started off that I had problems with my menstral cycle. It was lighter than normal. I took 5 pregnancy test within 3 weeks of last having sexual intercourse. They all came back negative. I have been having problem with my insides burning which feel like heart burn, problems with going to the bathroom either stopped up or going more than normal, and the top part of my stomach is very bloated and kinda tight. I take Prilosec OTC which seems to help, but I was trying to find out if anyone has had any problem like this and what it could cause. Any ideas or suggestions would be greatful.