Bella Thorne's Exes Mod Sun & Tana Mongeau Scoff at Girl Code Tweet

Mod Sun says Bella Thorne getting up in arms over girl code is ridiculous ... because she doesn't play by the rules in any of her relationships.

The rapper was out near Hollywood Thursday when we asked about his ex-girlfriend beefing on Twitter with her ex-girlfriend, Tana Mongeau -- who's also sort of his ex-GF -- and he clearly sides with Tana.

For the uninitiated, Bella dated both Mod and Tana -- at the same time -- and all 3 were cool with the arrangement ... until Bella had breakups with both earlier this year.

Then, after Mod was spotted getting dinner with Tana this week, Bella tweeted she was "no longer good" with Tana ... because she broke girl code.

Mod tells us this new rift is definitely about him hanging with Tana. As we've reported ... things haven't been good between Mod and Bella since their split, so it seems pretty clear she's upset Tana's still cozy with him.

Mod thinks that's totally unfair, though, because he's close with Tana from their time together while he was also dating Bella ... and he says they also worked on music together.