Dej Loaf checked someone during a BET soundcheck, and the guy ended up in the ER.

The rapper, who's nominated for 3 BET Awards and scheduled to perform, was at rehearsal Saturday at the Nokia Theatre when 2 of her former managers showed up unannounced.

We are told Dej Loaf and the hired help ended on bad terms just a week ago.  The 2 managers wanted to "talk it out" with her and, needless to say, it didn't go well.

Dej Loaf, her brother and her new management got into a scuffle with the old manager... leaving him with a cut forehead.

The manager went to the hospital for stitches.

You know what they say ...