Terrell Owens' Baby Mama -- You're in the NFL Now ... PAY UP!!!

Terrell Owens can easily afford $5,000 a month in child support thanks to his brand new NFL contract -- this according to one of his baby mamas ... who's now fighting T.O.'s request to reduce his financial obligations.

Owens had filed legal docs last year -- asking to reduce his enormous child support payments to FOUR separate baby mamas -- because he was unemployed at the time ... and seriously strapped for cash.

But last week, Owens signed a contract with the Seattle Seahawks -- a deal reportedly worth almost $1,000,000.

Which is why baby mama #4 -- a woman named Melanie Paige Smith -- has just filed papers, asking the judge to dismiss T.O.'s payment reduction request.

In the docs -- filed by Melanie's lawyer Randall Kessler -- she claims, "It is clear that a downward modification of [T.O.'s] child support obligation is not warranted" due to his new Seahawks contract ... as well as his income over the last 5 years, which Melanie estimates to be around $29,000,000.

According to Melanie, T.O. earned $7,000,000 playing for the Cowboys in 2007, signed a $13,000,000 contract with the Cowboys in 2008, signed a $6,500,000 with the Buffalo Bills in 2009, and signed a $2,000,000 deal with the Bengals in 2010.

And then there's T.O.'s indoor football contract with the Allen Wranglers -- which Melanie says was six figures.

But Melanie's math doesn't exactly add up -- T.O. was released from the Cowboys early, got injured mid-season with the Bengals, and was released from his Wranglers deal after less than a year.