What it is, is tendonitis. There are some things you can do:
--If it's your right arm, move your arm in front of you as if crossing your arms, only take your left thumb (as your left hand is palm toward your body), and deeply massage the muscles/tendons on the top of your right arm--right about on the pad, just down from from the crease of the elbow joint. It should hurt like the dickens if you do it right and it's inflamed!
Do it deeply and properly, and you'll find the middle finger on your right hand twitching as you massage. Do that regularly to reduce the inflammation and work the knots out of the muscle.
--whenever you knit, use a support that you can get for tennis elbow--just a band to go around the arm just below the elbow. That helps support that area while you knit. Something like this.
(We're a family of musicians and knitters and we all have these supports!)
--take breaks from knitting every few rows, or however often you need to.
--you can ice it as well!
--knit on a pillow in your lap to support your arms
--Try something like Icy/Hot or BenGay. I use a natural product called Sombra that I get from my chiropractor. That sort of thing will bring relief.
--don't forget typing and other repetitive motion activities will exacerbate the problem.
The main thing is to let it heal before you knit very much at a sitting. My dd was warned she might never play her flute again, but followed her instructions and is good as new--just has to be careful. I have flare-ups every so often, and follow these instructions--it works great.