I'm sorry you're hurting, that's so frustrating! I had the same problem after a major knitting marathon trying to finish a sweater. What helped me:
1. Take breaks and stretch about every twenty minutes, I make myself stretch every commercial break if I'm watching TV. (be careful of movies, I knit straight through LOTR: The Return of the King, OUCH!)
2. Try to have projects with different size needles, it makes you use muscles slightly differently.
3. If you can, learn to knit continental. I started out English, now I can switch back and forth (my tension's the same with both now) within a project, but even if you have a project being knit english and one continental it really helps.
Hope you feel better soon!