The rapper -- who's one of several artists performing alongside Omarion, Soulja Boy, Ashanti and many others as they tour the country -- went on an epic rant early Saturday morning ... just mere hours before he and the rest of the crew were due to perform.

As to what he was mad about ... well, it sounds like some internal drama backstage.

BW wrote, "Too much bitching and complaining on this tour! Im bussn cats asses EVERY NIGHT! Every night they try and complain and take something away from me!" He went on to add that tonight (Saturday) might be his last show as a result of this frustration.

Of course, his gripes come off as kinda vague ... so he clarified even further, and sure enough -- it all boils down to his setlist and how long he's allowed to do his thing in front of fans.

Bow Wow groused, "Heres the truth! I do 15 min EVERY NIGHT! I can't even do my full songs. AND I STILL BE BUSTING FOLKS ASS! think the fans don't complain about my set being 15 mins? Only way I show up tonight if I do my full set! Straight up!"

He wasn't done venting just yet ... Bow Wow then took to IG Live to rave some more -- claiming he was the brains behind the whole 'Millennium Tour' concept ... adding that Omarion and everyone else are really opening up for him. See Ruffin telling Otis the same thing here.

In the end, it sounds O got a hold of Shad and they talked it out ... because, before long, Bow Wow had changed his tune, promising ticketholders that he was gonna give them the show of his life, and seemed to indicate he'd be performing well past 15 minutes.