Am I wrong?

My wife has gotten to where she expects me to rub her feet with lotion every night. I've explained to her that sometimes I just don't feel like doing it (like if I'm tired and just want to relax), but when I don't do it, she will say something like "oh, I'm sorry rubbing my feet is such a hle" and then I feel bad about it. It's really starting to feel like a duty that I'm expected to perform on a nightly basis, and I don't know how to explain this to her without sounding mean or selfish. Anyone have any advice?