Sounds like an over protective gramma. It would bother me too, but thankfully (or hopefully) you don't have to deal with Gramma that often. Maybe make a joke about it to mom and see what her response it. "gramma must have thought I was laying around all day.... ha ha ha"

At the end of the day... you know what you've been doing all day. You know how hard you work and if you want to walk around in a moo moo then that's your right!