Ozzy Osbourne's Mistress SUES Kelly Osbourne!!

Ozzy Osbourne's mistress, Michelle Pugh, just dropped a bombshell lawsuit on Kelly Osbourne -- in which she admits to a 3 year affair with the rock star ... but denies she abused him.

Pugh's pissed over several tweets that Kelly fired off after Ozzy's secret affair went public.

If you recall, Kelly blasted Pugh -- a hairstylist -- saying, "Anyone looking for cheap chunky LOW-lights a blow out and a b***job call [phone number]."

She also accused Pugh of manipulating Ozzy -- saying, "My father is almost 70 ever heard of elder abuse?"

Pugh says she was "slut-shamed, bullied and harassed."

In her suit, filed in L.A. County Superior Court, Pugh claims the relationship was no fling -- that Ozzy made advances and turned their friendship into a romantic situation in 2013. She claims they secretly dated until May 2016.

Pugh says Osbourne's accusation that she committed elder abuse is not only wrong -- but defamatory -- and caused her all sorts of problems.

Pugh says she's been emotionally damaged and lost business as a celebrity stylist.

In her suit, Pugh also says Ozzy and Sharon did not break up due to infidelity -- it was their arguments involving financial issues and control.

She also claims Ozzy did NOT financially support her.

We reached out to Kelly's camp -- so far, no word back.

Sharon went on "The Talk" Wednesday, and discussed Ozzy's sex addiction.