I had this a couple of summers ago, and ice, rest and strengthening my hips really helped. Apparently weak hips really have an effect on your knees. I've been good ever since.
I agree to see a doc first though to be sure that's what it really is.
You only fail when you stop trying.
"The cure to anything is salt water: sweat, tears or the sea." - Isak Dinesen
5K PR: 25:28 (Nov. 09)
5 Mile Race PR: 44:36 (Dec. 2011)
10K PR - 58:03 (Oct. 09)
15K PR - 1:29:59 (Apr. 2011)
Half Marathon PR - 2:03:39 (Apr. 12)
Marathon PR - 4:48:55 (May 2011)