Need advice with knee injury?

I've had a lingering meniscus tear for a couple years. I can run on it fine, just causes me occasional pain, but not enough to warrant surgery. Anyway, a few days ago I sat cris-cross for the first time in a while. Since then, my knee's been pretty swollen and I haven't been able to run on it. I can walk fine, and without pain, but when I try to run, it's just too stiff and throbs afterwards. Any advice?