NEW: Nicki Minaj -- Don't Be Bitter About My Glitter

Nikki Minaj posted a picture of the crazy diamond ring her BF Meek Mill gave her -- and then got Insta-blasted by Floyd Mayweather’s girlfriend ... who has the same ring!

Then Meek started ripping into Floyd's GF ... and then it was basically a bunch of “messy, messy ring mess.“ Y'know how THAT goes.