4:35 PM PT -- The fight happened Monday night, and we've confirmed police responded to Tommy's Calabasas home around 10:45 PM. No arrests, but officers took a report.

Tommy was transported to a hospital where he got checked out.

Tommy Lee's face looks like he needs to see Dr. Feelgood, stat -- he says his son, Brandon, gave him a fat lip ... but we've learned Brandon's saying he was defending himself when it went down.

Tommy's mouth is swollen and bloodied in a pic he just posted, and he's laying blame squarely on 21-year-old Brandon ... one of his sons with Pamela Anderson. Tommy says, "My heart is broken. You can give your kids everything they could ever want in their entire lives and they can still turn against you."

It's unclear what sparked the father-son beef, but a source connected to Brandon tells us Tommy was drunk. We're also told Brandon absolutely acted in self-defense.

Also, on Monday Tommy took a shot at Pamela's recent interview with Piers Morgan, where she talked about Tommy abusing her. He responded, "Think she'd find something new to discuss instead of rehashing old s**t."