Kinda angry. Yourgrandma was wearing my thongs when I got home (erm...AKA flip flops for all you non-aussies, not the underwear). She knows how much of a germo/sweato/dirtophobe I am but she does it anyway
Even with the explanation, I still pictured your mom wearing your thong underwear, which would be at least a hundred times worse.
Flip-flops get pretty dirty when you wear them outside, anyway. Maybe you could buy your mom her own flip-flops so she won't borrow yours anymore.