I just got done with a practice session on the pipes. I've was playing some more advanced stuff from when I was learning with my teacher. I'm finding that my arms and hands are getting tired a lot. Either my reed is too open (which may be the case due to the recent humidy and heat), or I'm so out of practice that I'm applying the "piper's death grip."
The "death grip" is the tendency for new pipers to not relax when playing. Tense muscles tend to lead you to overexert, and eventually you start getting sore.
I find at this point, it's best to take a break, and come back in a little while after you've relaxed. I normally come back a little while later, take a deep breath and start playing again. This time I try to keep track of how tense my muscles are. I try to relax as I play, and normally it helps a bit.
I used to be able to play for a long time before getting sore, so the more I practice, the more my body will relearn what to do.