There could be various reasons. I would go and get checked over by your doctor as it could be a simple problem to fix or more complicated. In the meantime you could try to eliminate the following possibilities.

1/ A food allergy sounds like the prime suspect. Start to eliminate things one by one from your diet to see if they make any difference. You could have developed gluten intolerance or even intolerance to chocolate for example.
2/ What is the quality of the water you are drinking? Is it out of a tap or out of a well or a private source? You could try drinking bottled water to eliminate possible water contamination.
3/Maybe a reaction to antibiotics or other medicine if you are taking any.
4/ Bacterial infection in the gut etc. but you will need specialist help to identify that.

Good luck and let us know what it is when you eventually find out.