MediaTakeOut.com just learned an EXPLOSIVE piece of information. By now you may have heard that Young Money rapper NICKI MINAJ is no longer joining Rihanna's summer tour.

Well now we got all the JUICY DETAILS behind what REALLY happened. According to a VERY CREDIBLE snitch, Rihanna has been growing increasingly worried that her popularity is starting to fade. The insider explained, "No one [in Rihanna's camp] is panicking yet, but there is DEEP concern that Rihanna has peaked, and is on the downside of her music career."

Well when Rihanna learned that HOT AS FIRE new rapper Nicki Minaj was being included in her tour, she was FURIOUS!!! The insider explained to MediaTakeOut.com, "Nicki [Minaj] was brought in to sell tickets, but Rihanna did NOT want her on." So the insider claimed that Rihanna, presumably fearing that she would be upstaged, used her influence to REDUCE Nicki's role in the tour. Nicki was reportedly going to be the opening act, and be limited in the number of songs she could perform.

Well Nicki's people weren't HAVING IT!!! A person close to Young Money told MediaTakeOut.com, "Nicki is not an opening act, and we would never put her out there like that"

But that's not it. According to our Young Money insider, Nicki ended up calling Rihanna DIRECTLY to ask whether she was behind all of the tour drama. The insider told MediaTakeOut.com, "I only heard one side of the conversation, but [there] were a lot of curse words thrown back and forth."

DUUUUH-RAMMMMMAH!!!