At nine weeks, your muscles and tendons that were stretched and stressed by the surgery are still recovering their tone and strength. Even if your flex range is very good, you'll have to keep on rebuilding muscle strength by having your new knee singly support the full weight of your body to move it up and down, as when using stairs. There are exercises for this. I hated these at first because they were uncomfortable and hard to do at at the start. But they were the best home exercises for regaining knee stability, done 2-3 times a day at 10-15 reps each direction.

Stand with your feet together on the last stair step down, facing the wall, hands on the rail or flat against the wall for support, TKR knee on the up side of step. Bend your TKR knee and try to tap the floor below you with your good toe before bringing the good foot back up beside the TKR foot. You don't have to actually touch the floor below (or a stack of books or board to make it easier) at first, just make your knee do the work of lowering and lifting your body. Do this also to the front, facing down stairs tapping your good foot down to the front, and then another set facing up the stairs tapping your good foot behind you and down.
I think I was given this exercise during the last two weeks of PT, and I continued doing it at home for another month until I had no more issues or difficulties going up and down stairs. I still do it once in a while to check on my knees.