It is very common that a knee is difficult to bend following a cast. This used to be extremely common when it was the treatment of choice for dislocated patella's. Now I am happy to say that this is not the case. I wonder why if you could not bend it that you needed to go back into a cast? Seems a little weird.

For sure you will need rehab to get the leg moving following the cast removal. Perhaps if you don't have the money or access you could begin with working in the pool, even just trying to walk around in the pool if you are ok on the safety side of things. You can always get a flotation vest like that used for water running to help if your independence is a little challenged in the pool.