You've just had surgery - you have to expect pain and swelling.
If you've been immobile for a while you have to expect some muscle atrophy - so expect lots of catching/crunching etc. Even a short period of immobility will cause your muscles to weaken and therefore things won't be the same.
As you've had surgery, i'm sure your OS has referred you to PT's who have given you some exercises to do, to help the situation - quad crunches etc. These PT exercises are very important, but also remember that you have had surgery and it is going to hurt.
All the best