Burning pain in stomach and back

Hello, I am a healthy vegetarian who exercises regularly (6 days a week). I get period episodes of burning sensation throughout my entire stomach and mid-lower back region. It is usually not right after I eat, for example today I had breakfast at 7:30 AM and the pain started at about 9:30. Breakfast was baked yam with soymilk, wheat germ, and protein powder, about 400 cals. It usually lasts hours and antacids do not help. I have no idea what it is, but it is extremely uncomfortable. I usually just want to lay down and try to sleep until its over. Any input would be appreciated.