Wouldn't be surprised if you stressed some tendons or strained the muscles in your forearm. Too many revs, thrown too hard?
Lots of rest is probably called for, with a couple of Advil/Motrin/Ibuprofen every 4 hours. Not sure if a heat or cold wrap would be best. Probably heat.
Might be a good idea to wait for Sawbones (he's a real doctor) to comment. or just go to your MD or orthopedist tomorrow morning.
--------------------
"...for advice is a dangerous gift, even from the wise to the wise...."
J. R. R. Tolkien