Hi Alex -- I am not a doctor, but when I had the symptoms you describe, at its worst when going down stairs, steep hills, or when I put weight on it such as
in a lunge or squat, mine was caused by a stress fracture
in my tibial plateau.
I would suggest getting it looked at soon. It's been 9 months since mine occurred, I was non-weight bearing to partial weight-bearing for 12 weeks, and I'm still having problems.