I've read diarrhea can be a side effect of antibiotic use, such as with clindamycin, but also that a more severe/dangerous form can sometimes occur. how do you know if what you have is normal antibiotic-related diarrhea or a more severe form?

Symptom severity : A few loose stools a day usually is no problem. Get concerned and contact the doc who prescribed the antibiotic if many watery stools, cramping, fever, or if you see blood in stools.

Normal vs Abnormal : Watery/loose stool with cramping can be normal. This should resolve within 7-10 days after completing antibiotic. As mentioned, severe abdominal pain, new onset fever, blood in stool should prompt you to have your stool tested for C. Diff, a bacterial disease, and to stop the antibiotic immediately. You also should take a daily probiotic, but not at the same time as the antibiotic.