Okay but since when is walking home with a bag of groceries a bad thing?

I went to the shop and all my stuff didnt fit in my back pack so I just go a grocery bag to carry the rest. I walked about halfway home when I go to pass these two chicks an one of them literally says out loud to my face "Who walks down the street with a bag of groceries these days."
I HAD ONE BAG. WHY DO YOU EVEN CARE? PISS OFF...... iS what I would've said if I wasn't terrified confrontation.
I was just like "Cause I live like five minutes away." And they laughed together and walked away.
WHY? WHAT DID I DO? IM ALREADY A VERY SELF CONSCIOUS PERSON. WHY YOU DO THIS TO MEH.
Ah this is a bit long, ah well. Just some rant.