It was my groin but now it's my knee

It sounds like the punchline to an old joke. Anyway, the groin thing is gone, but it's been replaced by more disturbing knee funkiness. The lateral (think that's the term) side of my left knee bothers me sometimes on uphills and the back of my knee seems too lose. I'll occasionally hyper extend my knee, like when standing on my left leg to put on my pants, and get a sudden sharp pain. I did a really slow 8.6 miles today as a result. 9:38 pace and 132 bpm avg heart rate.