Arian Foster Baby Mama -- I'm a HUUUGE Gold Digger

The woman carrying Arian Foster's fetus is a gold digger ... and to prove it, she DRESSED UP LIKE ONE FOR HALLOWEEN ... and TMZ Sports has the photo.

Sources tell us Brittany donned the outfit for Halloween 2012 -- she posted the photo to her Instagram page along with the caption, "Now I ain't saying she a golddigger ... no wait yes she is."

Of course, the pic is interesting since Arian filed legal docs painting Brittany as a shameless fame whore who's trying to exploit the pregnancy to get a reality TV show.

And there's this ... Arian isn't the first pro athlete Brittany's been linked to. We know she was tight with NBA star Chase Budinger, who played for the Houston Rockets at the time.

In fact, she visited Chase in San Diego back in 2012. Unclear if they're still friendly.

She ain't messin' with no broke ...