This hugged-up pic with someone who looks a little like Bre Tiesi -- although, we're told it's not her -- included the caption, "I said I would never do it again but…Finally doing what the world wants me to do…"

His post also had a shot of a massive diamond ring.

The post threw Lil Jon and even Nick's former publicist for a loop in the comments section ... especially since Nick recently admitted all his baby mothers don't always see eye to eye.

Of course, Nick being the ultimate hustler of promotion, is more likely simply setting the stage for his "I Do" single rollout, which features Chris Brown.

Released in May as part of his latest album "The Explicit Tape: Raw & B," the song opens up virtually the same way with Nick admitting his dreams of marriage haven't completely faded and Breezy crooning heavy in the melody.