50 Cent -- Congrats On Your SECOND Child ... At Least According to Cops

The woman accusing 50 Cent of domestic violence is NOT Shaniqua Tompkins -- the mother of the rapper's 16-year-old son -- it's ANOTHER baby mama, TMZ has learned.

We broke the story ... cops say the victim of the domestic violence incident is a former girlfriend who has a child with 50.

50 has never publicly acknowledged having a second child ... but it appears the woman behind the allegation is Daphne Joy -- a model who famously dated 50 for 3 years.

There were rumors the two had a child last fall ... but neither 50 nor Daphne ever confirmed the birth. In fact, the L.A. City Attorney seemed to break the news in its statement to the media when it said the victim and 50 had a "child in common."

We spoke with Shaniqua, who swore she wasn't the accuser -- and we know Daphne lives in Toluca Lake, where the alleged incident took place.

So ... congrats???

Calls to 50 have not been returned.