It's Tuesday, and I haven't called my doctor yet because my knee keeps feeling better.
There's a little discomfort when I bend my right knee a lot, but it has improved. I'm hoping, as blackbeltblues suggested, it's just tendonitis or something.
I do hope I will eventually be able to sprint regularly--it does seem like the best way for me to cross train for conditioning and leg strength. But it's good that for now it doesn't seem like a serious injury.