Hi and welcome,
It is actually normal to have your knee do that. That is why you should use your walker until you have some strength built up in your leg. My leg did the "flamingo" (hyperexteneded) several times in the first few weeks. It was really unnerving, but as your quad starts firing things should become more stable down there.
As far as exercises, at this stage, you should be doing gentle heel slides, and straight leg raises.Donn't be upset if you cannot do them completely. Your leg has just been through a significant trauma, so things are not working the way they should. This resolves with time.
My IT band also felt horrible. I had my husband GENTLY massage the area with arnica cream. (Arnica also helps with the bruising).
Again, time, rest, gentle stretches, pain medicine, ice, elevation, and PATIENCE.
best,
Sandy