There are a couple of things that can possibly contribute to muscle pain after kidney donation surgery:
--- Sometimes the position our body was placed in during the surgery may have put a strain on some muscles.
--- After the surgery we may be up-and-around, but at the same time unconsciously changing our posture when we walk, sit, stand, etc. to avoid pain at the incision(s) or elsewhere. Changes from our usual posture put strain on different muscles that we don't normally use in that way and pain can result.
Doing some gentle bending and stretching exercises may be helpful, and also being conscious of getting back to good posture. But don't be shy about letting the hospital now if pain is bothering you.
Please let us know how things go.
best wishes,
Fr. Pat