TMZ has learned that on Britney Spears' last trip to Miami, prior to checking into rehab the first time, the turbulent pop diva appeared "out of it" while shopping at a local BeBe clothing store.

A source tells TMZ that Brit handed over her kids to the store's salespeople so she could peruse the racks of clothes. The gal's got priorities! When it came time to try the items on, Spears didn't bother with a pesky dressing room -- and stripped down right in the middle of the store -- in front of staff and other customers! We're told the "Toxic" singer even tried out some new dance moves for her shocked audience.

Not long after Britney's impromptu boutique performance, the mother of two checked into rehab three times and shaved her head. You may have heard about it.