Credit: Jesse Grant/WireImage; Jason LaVeris/FilmMagic

Bunnies don't always stick together.

After Crystal Harris nixed her Sat. June 18 wedding to 85-year-old Hugh Hefner, she threw a pool party in Vegas (with Heidi Montag) as her plan B -- but fellow Playboy Playmate Holly Madison really, really didn't approve.

"That's a new low!!" Girls Next Door alum Madison tweeted of Harris' Saturday appearance at Wet Republic at the MGM Grand Hotel.

When celeb writer Robin Leach also chimed in about Harris' wedding-day bash, Madison sniped: "That's disgusting and whoever booked her is tacky!"

Madison's critical Tweets did not go unnoticed by newly single Harris. "Hard times show your real friends..." Harris wrote.

"No," Madison retorted. "They show HEF's real friends..."

"It was too close to home in L.A.," Harris, 25, explained to Radar Online of her surprising Vegas appearance. "I just want to relax and not think about the fact that this was my wedding day."