Brit showed off a "Piece" of her toned body when she donned a tiny bikini over Labor Day weekend in Sin City -- where she ran into ex-paparazzi boy toy Adnan Ghalib!

Fortunately for the pop mom's newfound health, "Extra" knows that the former flames did NOT rekindle their trainwreck romance -- although People magazine reports that Brit and Adnan came within a few yards of each other several times.

What doesn't happen in Vegas, thankfully stays in Vegas.

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