Justin Bieber and Selena Gomez headed to the Hard Rock Hotel & Casino in Las Vegas this weekend to watch his pal Floyd Mayweather Jr. fight Marcos Maidana at the MGM Grand Garden Arena, but the 22-year-old singer didn't spend the entire weekend with her man!

One of the singer's pals posted the sexy pool pic above, and the ladies looked like they were having a blast soaking up the sun. After Biebs got back from lunch he reunited with his lady love, and it looks like they had plenty of alone time -- the pop star had a three-bedroom, penthouse suite at the Hard Rock, and the on-again lovebirds hung out at the Breathe pool lounge.

Oh, and speaking of bangin' -- the pic she just posted of her new hair? Loves!