Check out this footage we got, which we’re told was taken very early Saturday morning at the Skate Rock City rink in Vegas… as CB was celebrating his 34th birthday among lots of celebrity friends, including Usher – with whom he is incredibly close.

While Usher and co. He sang to Chris earlier in the evening when all was well, unfortunately things got worse in the wee hours of the morning… when eyewitnesses told us Chris was trying to talk to Tiana Taylor, who was also present and sitting on a bench outside the rink. We were told, for whatever reason, that Tiana was ignoring him.

Our sources tell us Chris was angry about it, and started yelling at her…that’s when Usher stepped in and tried to calm him down. You can see it in our video here.

We’re told Chris didn’t have it, cursing Usher and Tiana — but then, he focused his anger squarely on Usher. Eventually, our sources say Chris told his crew they were leaving and walked out of the facility. Usher was told to take off his skis and chase after them.

An eyewitness – who said they were present for this – told us that they saw Usher go behind a series of charter buses that were parked in the lot…where Chris and his crew were at. Shortly thereafter, we were told, Usher showed up with what appeared to be a bloody nose.

It is unclear if he went to hospital, or if a police report was filed. We’ve reached out to Metro Vegas – but haven’t heard of anything remotely resembling this.

Meanwhile… Usher, Teyana, and Chris have been silent on the radio this morning — and none of them have said anything yet to confirm or deny a report initially made by Hollywood unlockedwho broke the news of a fight between two R&B stars/friends.