i had that same problem, but not just in my picking arm/hand, it was very severe in my left hand while performing sweeps, i got into a bad habit of twisting my rist whilst doing the hammers/pull-offs to certain sweeps, well it got so bad i had to stop for a while, and cudnt play for a bit, so i sat down and went through EVERYTHING slowly, and pin pointed where the problem was, well got it sussed and re-learnt the technique, with the correct hand postures, and *touch wood* ever since i've been ok :) So it may be worth reading up on correct posture, and MAKE SURE YOU WARM UP, that is a MUST DO, specially if you play speed licks

Hope i didnt go on too much :o