Minor cp. Tense only when nervous. Weigh 115, 5.5' doc told me that my first pregnancy will be c/s. Why? B/c of my tensing? Sm pelvis? Wont it expand?

Individual case. All of those are good guesses. You will have to ask your doctor. Just because one has CP does not mean a vaginal delivery is out of the question. But in your case all of the things you mentioned would need to be assessed.