Originally Posted by a2psyklnut

Not to piss on everyone's parade, but chances are since you are new to mtn biking it's probably just "USER ERROR". Most likely you've got the "DEATH GRIP" on your bars.

Little tip. Rest your pointer and middle finger on the brake lever at all times. When you're riding a smooth part of the trail, don't wrap your thumb around the bar. Rest it on top.

You're getting what the MXers call forearm pump. For them it's due to twisting the throttle and trying to control a couple hundred lb bike. For you, it's inexperience! (I'm guessing)