Health Problems

So basically I've been having problems eating and loosing weight for several months now. I cannot eat a lot and certain things I can't eat all together (sweets, Pastas, some breaded things, pizza. Veggies are one of the only things that are fine) When I'm having a bad day with the food, I get a migraine, pain in my lower left stomach, and really tired and nauseous. Because of these problems I've been loosing a significant amount of weight. Along with those things, my periods have become much heavier, longer, and painful! And I've been moody constantly! I'm just wondering if anyone knows what could be wrong or if it's happened to any of you.