iarrhea is the body's evacuation system. If it doesn't like something you put in your stomach pain after eating
, it will get rid of it.
This being said, it sounds like you may have a food intolerance; they're surprisingly common and can start at any time. Take a look at the meals you've been eating over the past few weeks. Do they all contain a certain kind of food or ingredient? If so, try an elimination diet for the next few weeks and see if that helps.
If it doesn't, there's also the possibility of irritable bowel syndrome or a helicobacter pylori infection. If the elimination diet doesn't help, I'd recommend seeing a doctor.