Could be so many different things or combination of things, I'm not really suprised you didn't guess it right off. That's all we can really do from here is take an educated guess. Some problems , if described well ,can be nailed down from a distance others you can only guess at and start with the easy stuff then start on the most likely of the more difficult.

I learned all I need to know about life by killing smart people and eating their brains.
Eat right ,Exercise ,Stay fit, Die Anyway!