Medial collateral ligament strain. Fell off a windsurf board. Left leg went down to the sand, right foot got caught on a footstrap, leg got pulled out sideways then as it wouldn't go any further out sideways, leg twisted and knee bent, straining the inside side of the knee. Banged it on the mast at the same time so it got bruised and strained together.
Very painful for the first 24 hours - couldn't bend without pain. Iced and ibuprofened. Now, 1 week later, it's still sore. Still hurts to twist the R leg, but walking and cycling are fine (as long as it doesn't get twisted).
No real wisdom here. Iced it quite a lot and didn't do too much stuff to aggravate it (apart from two O races).