How long should a side stitch last? Still feeling one more than 12 hours after exercise. Should I be concerned?

Hard to say w/o exam. A stitch will usually go away pretty quickly after stopping exercise. I recommend calling your doctor. It's really not possible to give advice about an individual's symptoms online. Your doctor will probably ask you a number of questions to determine whether you need to be seen.
No. Could be over exertion or a muscle strain, but waiting 24-72 hours is not unreasonable. Try an anti-inflammatory medicine and some ice. If you are still having discomfort after that, then see your doctor.