Nicki Minaj's retirement announcement not only came out of nowhere, but it absolutely doesn't add up -- as in it doesn't seem real -- based on her recent actions ... so say sources close to her.

TMZ's learned Nicki has been busy in the studio working on new music in recent weeks and on top of that ... we're told she made plans to record with several artists just in the past few days.

Our sources say Minaj has a lot of songs already in the can, and despite saying on Thursday she's retiring in order to "have my family" -- she still has studio time booked until next month.

So, why the sudden decision to hang it up? We're told Nicki's been growing increasingly fed up with internet trolls who've been harassing her lately, and finally reached a boiling point.

Our sources say the rapper was pushed over the edge by constantly defending herself over accusations of not writing her own songs, questions about her hip-hop legacy ... and her relationship with her fiance, Kenneth Petty. BTW, rumor is she's already Mrs. Petty, but that's another thing Nicki's tired of discussing.

The good news for the Barbs ... we're told those close to Nicki believe she's just taking a much-needed break from the haters and simply needs a moment to chill and reevaluate. As we told you ... DJ Paul agrees.

One more thing -- Minaj's comment about family got many wondering if she's pregnant. We're told she's not ... yet.