Arm pump. That's what the yanks call it. Dirt Bike and MXA have had articles on it. It may not be your case. They say in the following article it may be due to many causes including carpal tunnel syndrome, ganglion cysts, arthritis, neck abnormalities, tennis elbow and fractures. I had a similar condition when I was 16 but it went away with time. Here's the article:
http://www.motocrossactionmag.com/me2/dirm...6C914E5A0E9FAF8