It sounds like to me that you may have some osteoarthritis forming within the medial compartment of your knee and and at least and X ray or (preferably andMRI) would be able to correctly diagnose your knee Alternative provisional diagnoses that come to mind are hamstring tendonitis, cystic degenerative changes of the medial
meniscus.
A correct diagnosis would guide the exercise advice but I would advise knocking off the 5kg leg lifts and replace with prone eccentric hamstring exercises for starters (your physio should be able to show you how to do these). Keep up with the balance exercises roller blading and swimming and cycle <1.5mile.
let us know how you go with your knee!