Selena Quintanilla's Brother A.B. Apologizes After Chastising Festival Crowd

Selena Quintanilla's brother, A.B., III is offering up an apology after losing it on fans this weekend during a music festival in Texas ... and he's blaming the fact he was off his meds.

In a since-deleted video shared to Instagram, the musician -- who used to tour with his late sister -- said he regretted his behavior at the Tejano Explosion 2024 music festival, in San Antonio ... revealing he was forced to go off mental health medication amid a cancer scare weeks prior.

He explains ... "After Selena passed away, [mental health issues] came around. I became bipolar. I became OCD, ADD, PSTD [sic] ... I'm on the spectrum basically, for people who understand that."

As A.B. puts it ... when he isn't on his medication, he is prone to having breakdowns. He added ... "I broke last night. I said things that I don't mean, that I don't feel in my heart whatsoever and from the bottom of my heart I want to apologize to everybody."

This update came a day after A.B. sounded off on festivalgoers for their lackluster reaction to his set. The musician became noticeably upset during his performance, berating the crowd for acting like they were forced to be at his show.

It was an ugly scene ... and a lot of people were verbally sparring with him, and even booing. A.B. was later filmed being escorted to his tour bus by the police ... while frustrated fans hurled insults at him.

The artist is Selena's older brother ... who produced and wrote many of the late Tejano singer's hits. So, while his delivery may've been off -- he is due a bit of respect for his influence in the music industry. No doubt about that whatsoever.