Likely hood is you have pulled a muscle.
Seeing your doctor is a good idea, or a Physio.
If it's a pulled muscle, you would want to bandage it, keep it warm. Also try putting it in warm (NOT HOT) water, and moving it around, if it doesn't hurt as much then it would more then likely be a pulled muscle, muscle stress.
If it doesnt help, then you could have infact damaged the bone! A crack on the bone, could cause alot of damage in the long term. So its best you go to the doctor if that be the case.
Good luck with your arm, and Karate.