thanks everyone for the advice and well wishes!
I'm happy (ecstatic, really) to report that my knee is feeling MUCH better. Last night I swam and it felt fine.. the flip turns made my knee feel a little wonky, but I managed to do them with my left leg straight (that must have looked hilarious). This morning I really don't feel any pain, except when I bend or strighten it suddenly. I can walk normally without pain or limping.
RICE has really been amazing.
Rest: swam last night, haven't run since Saturday, and will not run for a while. May try cycling in a few days.
Ice: I've been icing as often as possible.. at home and at work.
Compression: Bought a cheap knee compression sleeve and have been wearing it regularly.
Elevation: when I can. currently my leg is propped up on my desk with a bag of ice on it. have been propping it up with a pillow at home in bed.
I think maybe it was just really inflamed? I'm not sure but I'm glad it appears to be healing quickly. I need to remember to continue resting. I have a strong enough running base -- from both IM training AND from an early spring marathon this year -- to not run for a while. I'd like to get in one more long ride this weekend but I'll see how I feel.
This week will be a lot of swimming.. which is good. I like swimming.