Offset's Alleged Side Chick Was in Cardi B's Archenemy's Music Video

Cardi B's breakup with Offset over alleged infidelity just got a little more dramatic -- 'cause the girl he's accused of creepin' with was in a Nicki Minaj video.

Turns out, Summer Bunni -- who Offset allegedly solicited for a threeway with another female rapper -- starred in Tekashi69 and Nicki's "FEFE" music vid. SB posted a clip of herself in the production shooting whipped cream in her mouth.

That's what they call a star turn in show biz.

As we reported ... Summer says she feels terrible about possibly being the reason Cardi and Offset split, and tearfully told us she never meant to be a homewrecker.

It's unclear if Summer was trying to stick it to Cardi by working with her nemesis. The "FEFE" vid came out in July -- meanwhile, Nicki and Cardi's bad blood boiled over in September. So, this could all be coincidence, but drama-filled coincidence, for sure.