Alright this is just a suggestion. But there is a ligament attached to your knee when your younger, and as you age it detiorates, but for some people it doesnt. It can cause un- noticable to severe pain when working out. During my knee surgery i had it cut, and to put less strain on my knees i have been biking and mix in seated leg squats every so often. My suggestion would be to get an MRI to be completely sure everything in the area is a-okay and try biking with some seated squats. Go for high repititions at first.
Side note: After my leg workouts(15-60 minutes) I consume a we ll balanced meal consisting with healthy antioxidants because it is proven that they increase muscle and tissue healing and after a workout your blood is pumping the most giving the nutrients of your food to all of your body with maximum efficiency. I would also Ice and take 2-3 advil/ibuprofen after each leg workout. My opinion.
Good luck