The former Danity Kane singer -- who used to be signed to Bad Boy and worked extensively with Diddy in the 2000s -- shouted out Cassie on social media after her bombshell lawsuit against her ex went public, writing ... "Been Trynna Tell Y'all For Years."

She also told Page Six she's in "complete support" of Cassie ... and that what Cassie describes in her complaint probably echoes some of what Aubrey herself has said about Diddy.

If you're unaware, Aubrey's been doing multiple interviews over the past year or so ... in which she made allegations of her own against Puff -- but not quite to the level Cassie did. AOD's concerns mostly stemmed from what she claimed were shady biz practices ... and unwanted sexual advances from Diddy that she claims she had to rebuff over the years.

Aubrey also recently slammed Diddy finally signing off on his former artists getting their publishing rights back ... claiming that he had them sign NDAs as part of the deal.

She says she was offered her own publishing rights back as well, but refused to sign the alleged NDA she says came attached to it -- something else Aubrey's feeling vindication over.