It's very possible that it's a tricep exercise you're doing. Like the others suggested, see a doctor. A family prac should be able to tell what's wrong, and if not, they'll refer you to a specialist, probably an orthopedist (everyone in medical school goes on rotations so they have at least a little experience in every field).
Don't ever think for a second a doctor reads some books, and gets a license to practice medicine. They see patients in and out for years along with bookwork and cracking open dead bodies and a lot of other things you can't even imagine.
About anyone here being a doctor, give me 6 years to finish school...
