Paulina Rubio's Ex-Husband -- She Screwed Me Over ... Now I Want Revenge

Paulina Rubio's ex-husband desperately wants a piece of her "X Factor" fortune ... claiming the singer screwed him out of his rightful share of the spoils in their divorce settlement.

Nicolas Vallejo-Najera just filed legal docs, accusing Rubio of concealing financial info during divorce talks earlier this year ... stuff like her big money contracts with "X Factor" and another talent show on Telemundo.

Nicolas and Paulina settled up in March. Under the deal, Paulina agreed to pay Nicolas $6,750 a month in spousal support for 3 years -- but Nicolas now says that number was based on Paulina's low ball income estimates and now he wants more.