It could be an overuse issue. What did she do during the day? Have you thought about Tennis elbow, which is a tendon problem. If it continues, see a doctor.
Reta
Hi i'm not a doctor, but I decided I might be able to help. Now considering she is only 15 I wouldn't worry so much about it being a stroke, but you also can not rule that out. Even young people can sometimes get under so much stress that they can have a stroke. On the other hand this could be the same thing my cousin has had before. My cousin drank a lot of soda's and he started to develope a pain in his right arm, so they tookd him to the doctor and did all kinds of test. Come to find out he has had to much, sodium in his diet, from drinking the soda's. he had to lay off of the sodas and stop drinking so many of them, he drank mostly water and tea, and this made the pain go away by decreasing the amount of sodium in his body. The other possible cause of the pain in her arm could be from a simple strained or pulled mussle. It can cause mild to severe pain and come and go considering how you move you're arm. Usually taking an icy hot patch and applying it to the affected area usually helps to momentarly ware off the pain if this does not work then it could be that she may drink to many soda's, in any case I would take her to her normal physician to do a checkup just in case, not an emergency- unless needed. In the meantime I would lay off the soft drinks if she drinks any, and maybe try the icy hot patch. I hope i've been a big help, and contact me if you wan't any more information about the pain in the arm. thank you.