Re: Can corns/calluses be invisible? (and seasonal?)
Does her skin get dry in the winter? When I was a child, I used to get someting similar. My skin would get very very dry in the winter and I would get tiny, almost invisible cuts on the bottom of my feet. If I didn't use a dermatological cream at the beginning of winter I would get wickedly bad pains on the balls of my feet. It didn't look like there were cuts on my feet, and they didn't feel exceptionally rough or "cracked like" but there were little cuts.
If it's not dry skin or something similar, it may be a bit of arthritis at one of the many joints in the foot and the cold and wet of winter makes it act up, like how my knee gets worse when it rains.
I don't know really, but these two ideas came to me and I thought I'd share them. I hate seeing threads started and no replies.
Good luck to your mother.