Madonna is such a big star, you wouldn't think she'd care if someone didn't want to collaborate with her, just move on to the next, right?  Well, apparently, the Queen of Pop has taken offense at being turned Mother Monster.

While onstage Sunday night in St. Paul Minnesota, Madonna told the cheering crowd, "I recognize a lot of familiar faces and it makes me so happy to see you again!  I have the best fans in the whole world!  So take that, Lady Gaga!"

As the crowd roared its approval, Madonna continued, "You know, I invited her to sing with me onstage, but she turned me down."  As crowd booed, Madonna added, "It's OK. I've been rejected before. It's good to build character."

It's actually not clear if Gaga did, in fact, turn Madonna down, or simply wasn't able to join her because she's in the middle of her own tour.

Recently, Madonna was put on the spot by Ellen DeGeneres, who asked her what she thought of Gaga.  Madonna admitted that Gaga had a "good voice," and also praised one of her tattoos.