Weird Breaststroke Knee Event/Injury

I have been excitedly gearing up for the winter/spring swim season. But several weeks ago, at the tail end of a workout I was swimming some hard IMs and about 10 yards into breaststroke, at the snap of a kick, my right knee hurt like hell. I was unable to continue, and locking my knee was briefly impossible. The pain lasted about 10 minutes, then went away. Today, it happened again, but worse. Same sudden grinding pain, but a few hours later I am limping and afraid that I may go down if weight lands on my right knee the wrong way. The pain seems to be seated below (that is toward the bottom of, but also underneath) my kneecap. I'm icing it, and I am hoping it will go away tomorrow. Fingers crossed. Has anyone ever had this experience? I have never had a problem with my knees, ever. I'm 54, and so far no muscle/skeletal difficulties of any other sort. Both times it happened at least an hour into my swim, and both times it was when I was swimming especially hard. I swim IM a lot in workouts, because it's fun, but I am not much of a breaststroker. My best strokes are fly and free. So laying off the breaststroke in practice would be an inconvenience, but not nearly so much as hobbling around. I will welcome any anecdote, insight or guidance. Thanks.