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It appears pop princess Katy Perry and singer-songwriter John Mayer have fallen off of cloud nine and broken off their relationship for a second time. The break comes just seven months after the pop duo reunited.

A hint came from Perry, 28, that her relationship with Mayer, 35, had hit a rough patch with a cryptic, frustrated tweet from the "Firework" singer on March 18.

"ATTENTION: Mercury is OUT of retrograde today! Thank God!" Perry tweeted.

The couple's on-again, off-again romance began during the summer of 2012, about six months after her ex-husband Russell Brand filed for divorce. Perry and Mayer called it quits for the first time by August, but reunited in September, and were then seemingly inseparable.

"In the last couple of weeks people noticed that they have not spent much time together at all," Raha Lewis with People magazine told ABC News.

As recently as two weeks ago Perry happily tweeted, "My boyfriend is taking me to a kitten shelter in his truck … I can't think of a more perfect Saturday."

Perry confessed in her 2012 concert movie, "Katy Perry: Part of Me," that she longs for a romantic partner who isn't threatened by her success, which is leading some to speculate this may have contributed to her first split with Mayer.

"They won't be threatened or have weird motives," Perry said in the movie. "They will be accepting of me, the things I love, the ambition and what I aspire to."

Sources say, however, the pair was very compatible.

"They talked about music and wanted to watch all the Oscar movies together. The relationship was a romantic one," Lewis said.

Mayer, who is known for a string of A-list ex-girlfriends, including Jennifer Love Hewitt, Jessica Simpson and Jennifer Aniston, says he is maturing with his relationships with women.

"John Mayer recently talked about the fact that he's developed the capacity to love," Lewis said. "And it's with this capacity that he moved forward with Katy."

For now, their careers will keep them apart. Mayer is now gearing up for his upcoming tour, while Perry is back in the studio working on her next album.