What Causes Irregular Periods,bloating And Swollen Breasts?

Hi, I'm 18 years old and since october 2010 my periods have gone from being pretty regular to not at all - not coming for months, and then when they do they are very very light. This month I had a very light period, with usual PMS symptoms - but they have not gone away, for 3 weeks I have had constant bloating, swollen breasts, and pain in stomach that comes and goes - like as if I had overdone it on a run, with nausea - often after I eat. I have also started to experience some diarrheoa and gas problems. It's so uncomfortable. What could it be?