Many gave good advice for "tennis elbow," and i would also like to throw in my 2 cents as a Physical Therapist. Any sign of pain means you need to ice, ibuprofen and rest. Rest=NO KNITTING FOR AT LEAST 2 WEEKS! Yes, believe me, I know--I can't live without knitting that long. However, having had my own course of lateral epicondylitis twice, and NOT respecting my poor left elbow's pain the second time around, I now have chronic pain for 3 months. The first round, I actually did as I instructed--no knitting for 2 months! I then knit happily for a year with no problem. The second time I noticed the pain, but I HAD TO FINISH THE BABY GIFT! NOT! So, now I am suffering like a fool. So here are my suggestions:
1. First sign of pain: ice ice ice, ibuprofen as a steady diet for 3 days, and REST REST REST (ice pack on for 10 min at a time, and do it at least 4 times/day)
2. If you are a fool like me, you can consider PT, or acupuncture--both can be helpful if the practitioner is good, but you must still rest.
3. The elbow strap recommended helps the symptoms, but if you continue to knit, you are kidding yourself and setting up for some BAAAAAAD pain.
4. Did I mention no knitting? Rest? Yeah. Do it.
BTW, even if you don't have pain, you also should be taking breaks, stretching, changing how you hold the needles, and not knitting for 3 hours straight. Did I mention rest?
Before resuming your knitting, you need to do some slow, gentle strengthening, which is why PT can be more helpful than acupuncture alone.
Good luck! and don't forget the rest.