I've had what feels like stomach cramps for a few days off and on now. I've also had diarrhea one day. What could this be?

Gastroenteritis. If this is brand new, my first impression would be gastroenteritis. As long as you're not having debilitating pain, bloody stools, or frequent copious watery diarrhea, i would just watch it. If it worsens or lasts longer than 4-5 days, you might want to be examined by your doctor.