Sources tell TMZ ... Madison's boyfriend, Brett, popped the question and Madison didn't hesitate for a second with a "Yes," because, as one source put it, he treats her better than any man has before. She also loves that he's a good father-figure to her 8-year-old son, Hudson.

Our sources say Madison and Brett didn't meet that long ago and weren't even living in the same state when they first started dating.

We're told the relationship is so mysterious ... a lot of her friends haven't met Brett but she's ALWAYS talking about how much she cares about him. Back in April, Madison, who previously dated co-star Austen Kroll, teased on social media she was dating someone new. And, 2 months later, she and Brett went IG official.

The engagement comes not long after Madison was rumored to be at the center of A-Rod's split with Jennifer Lopez. Remember, there were claims A-Rod got cozy with Madison, but A-Rod's team said they never even met.

She had also previously been linked to ex-Bears QB Cutler... though that was a brief fling. Nevertheless, Madison now seems more than content to have a new man.

While we don't know much else right now about the relationship, our sources say Madison and Brett's relationship is documented in the upcoming 8th season of "Southern Charm" ... though it's unclear if production cameras captured the proposal.