Paul Ryan's Ex-GF -- Served Prison Time for Wire Fraud

Now we know why Paul Ryan's ex-girlfriend was so tight-lipped yesterday when we asked her about the VP hopeful ... turns out she was indicted by the Feds back in 1999 and spent months in prison for committing wire fraud ... TMZ has learned.

Deneeta D. Pope -- who made headlines recently as Ryan's African-American GF in college -- was indicted by a Grand Jury in November 1999 for allegedly swindling her former employer -- Ernst & Young -- out of $77,000.

Pope pled guilty in 2001 to stealing a lesser amount, but admitted she inflated expense reports, fabricated receipts and turned in phony invoices. She then used a wire transfer to move the money from a bank in Delaware to a different account in Chicago.

Pope created the scam by claiming she attended an educational course for work. Turns out ... the course didn't even exist.

Pope served 5 months in Federal Prison.