What's wrong with my knee?

2 years ago while hunting in the mountains my knee started making a clicking noise the next day it was painfull to walk on and I limped out. I havn't been in the mountains for 2 years so I whent for a climb, my knee seems to be ok on the flat but gets painfull whenever I walk up or downhill. I whent to the doctor, he pulled and pushed it then perscribed a course of ibuprofen that I have decided no to take as my knee is not in pain on flat ground. The injury is likely due to over use but what have I injured and how do I fix it?