I have an Ibanez GIO electric guitar and I'm trying to improve my picking technique to play 16th and 32nd notes(alternate picking). I've been playing guitar for 2.5 years.

Whenever I try to play something fast, my forearm seizes up within a few minutes, and once, after a particularly lengthy playing session, the region opposite my palm was throbbing, so much so that touching it was excruciating. I don't want to damage my body, and I feel that perhaps I'm tensing my arms too much. I also want to be able to play fast downstrokes (something like Master of Puppets) but it ends up hurting me even more. Any suggestions?