Hi there, like going to the gym...

Make sure you do some warm up excersizes.

GT has loads of warm ups, if you search for warm ups

Here is one..http://www.guitartricks.com/lesson.php?input=14209


Above all it might be worthwhile seeing a guitar teacher because it may be a posture issue that can easily be resolve, but left unresolved my cause long term frustration.

Cheers

Rob