Adrien Broner & Marcos Maidana Fight -- Formal Investigation Into Cheating Allegations

Trash talking boxer Adrien Broner got his ass beat and his welterweight title stripped in a stunning loss to Marcos Maidana this Saturday ... but TMZ Sports has learned, there's now a full-blown investigation into allegations of cheating.

Sources close to "the Problem" tell us ... the Texas Boxing Commission is now analyzing video of the fight which some say shows Maidana taking what looks like a pill during the WBA welterweight championship fight last Saturday.

The rules are so strict ... even water needs to be tested prior to a fight before the boxer can consume it. Pills and any other substance for that matter are off limits.

Broner -- who was previously undefeated -- says the loss hasn't shattered his ego ... he's confident the investigation will prove Maidana cheated.

We're told the boxing commission will reveal the results of the investigation in 1-2 days.