What could cause me to have what feels like coffee grounds in my stool?

Does it also look like coffee grounds? If so, there is bleeding in your upper GI tract. In any case, this is a medical question, not a food and cooking question.

Eating coffee grounds.

I doubt that there are any gastroenterologists or proctologists here on Y!A that could diagnose you via the internet. You show go see your doctor, for a 'hands on' examination.