From your description it's unclear whether this is done as a form of self gratification or she is merely scratching an itch, irritation, or whatever. You did say that this is not something she does in public, so it couldn't be a matter of public embarrassment on your part.

It seems to me that the relationship we have with family members is much more casual...perhaps even "unguarded"...than the relationship we have with others. Keep in mind, you mother changed your poopy diapers, so you certainly have few secrets from her. I guess she's entitled to scratch whatever itches her while in your presence.