Whenever I eat chicken I get a stomach ache and have to go to the bathroom. What could it be?

Practically everyone. ...Has some food intolerance. It's common, variable, and individual and may even include such innocuous ingestants as water. Reference: elaine shimberg, "relief from ibs". In your case, if chicken upsets you, don't eat it, or change the method of preparation and try again.
Fat in chicken . Unless it's the skinless breast chicken has a lot of fat. That causes release of hormones that regulate digestion. Some people are very sensitive to this and get that kind of reaction.
Sensitive. If the chicken is cooked completely you may have a sensitivity or allergy. Talk to.Your parents and your doctor about this, meanwhile avoid chicken.