Dwight Howard -- Baby Mama Sues ... You Screwed Me Out of a Fortune!!

$10,000 per month is WAYYY TOO LOW for child support when it comes to Dwight Howard ... this according to one of his baby mamas who says Dwight has been screwing her out of serious dough and now she wants to collect.

The Houston Rockets star is being sued in Florida ... by a woman who claims she and Dwight struck a private deal back in 2013 that required him to fork over $10k per month in child support.

But according to the lawsuit, obtained by TMZ Sports, the woman (Tiffany) says Howard hasn't lived up to his end of the bargain ... and since January, has only paid her HALF of what he owes.

Tiffany says the financial slight has forced her to reconsider their arrangement ... and since he makes more than $27 MILLION per year in salary and endorsements, she wants a bigger slice of the pie.

The kid is currently 4 years old ... and Tiffany says she needs the cash to put the child in expensive private schools, pay for extracurricular activities and fund social activities like birthday parties and sleepovers that will help the kid make friends.

Tiffany also says ... since the child has a famous father, she'll need extra cash for therapy to "deal with the unique issues involved in being the child of a celebrity."