I'm not trying to play doctor, but in any of these surgeries they cut the bone out and replace the joint with a metal one. The metal inserts look like a wood rasp and are pounded into the marrow part of the bone. It does not sound like an infection, but perhaps more of stress fracture in the bone or perhaps a muscle tear, which would account for the fluid as the body is trying to heal itself.
I spoke to a neighbor who had an infection right away in his hip replacement and he was in a great deal of pain as it took the doctors forever to figure out the problem. For your Mom the best news is, is she has you, that there is soreness instead of severe pain and you are going back right away for them to get things sorted out, as again they did blood work and if that did not how a white count, there is not an infection.
God bless both of you for her to heal soon.