Yep, I've got it a bit just now and pretty sure it was down to a particular hilly run I did. Physio also said I have a weak glute on same side and has given me exercise to strengthen it. But basically, it's just ibuprofen and rest for the knee, then work on any muscle imbalances/weaknesses to help protect the knee joint.
About 2-3 years ago I got severe knee pain (couldn't even run 1/2 mile) and went to a podiatrist who gave me orthotics. I have a leg length discrepancy which was putting undue stress on that knee. May be worth while you getting your gait checked out?