Cardi B and Stefon Diggs Argue Outside Maryland Coffee Shop, on Video

Cardi B and Stefon Diggs might not be as chummy as they looked over Mother's Day weekend ... because we've obtained video of them engaged in an intense debate in Maryland.

The video -- taken Wednesday afternoon outside a coffee shop and a gym in Burtonsville, Maryland, near Washington, D.C. -- shows Cardi aggressively yelling at Diggs ... who is leaned back, arms crossed, against his sportscar.

We can't tell what she's yelling ... but, she gets so animated that another dude with her comes up and puts a hand on her in an attempt to calm her down. A source tells us they overheard Cardi say "That bitch is messy" -- unclear to whom she was referring.

Our sources say this argument went on for a while -- at least 10 minutes -- and, while it wasn't that big a scene, security did turn away fans who wanted Cardi's autograph.

It's surprising to see these two arguing ... because everything looked fine between them over the weekend when Cardi showed up to support Stefon at a Mother's Day event for his Diggs Deep Foundation Mother’s Day.

Remember, Stefon kissed her on the side of the head and the two were hanging all over each other at the event ... with reports even coming out that they greeted folks at the party together, welcoming them to the soiree.

However, it seems they're more or less in the same place they were when they initially split earlier this year ... when Cardi threw serious shade her baby daddy's way.