I've had a ruptured Achilles tendon and damaged/dislocated both knees thanks to rugby. I tend to blow off shoulder pain but knee pain scares the crap out of me. I always rest up as much as possible when the knee hurts. it's one of the few things that'll get me to go back to the doctor. If my knees go, I'm screwed.
Go. To. Your. Doctor.
That is, if you have insurance and can afford one....