Britney's Mysterious Armpit Stain

Britney Spears it out and about Los Angeles on Saturday, wearing her favorite birdie fedora hat, carrying around a Red Bull energy drink and a walking with a mysterious armpit stain on her gray dress.

The pop princess also paid a visit to a furniture store near her home only to find it was closed.

Is that a sweat stain?? Or did she shove that can of Red Bull underneath her arm?? Usually if you sweat, both of pits sweat!