Could just have been a build up of lactic acid... happens when you push yourself too far. Not a good idea to play past the burn on the guitar man, you can end up damaging your tendons. My sister had to get shots into the tendons on her wrist (ended up getting a gangleon) after she did her instructors exams... not a pleasent experience. By all means get it checked out to make sure it's not neurological. If not, just take more breaks between playing!