try not to worry about getting exact diagnoses when you feel discomfort!
if you havent been in the gym for a while and then you go back, expect to feel it in the days afterwards.
there's the usual issues with knee pain of this ilk. have you had a biomechanical assessment? has your physio looked at your hip in detail?
also, what sports do you play? or did i miss that in my skim over your posts...