Ozzy Osbourne's Mistress -- SUES KELLY OSBOURNE ... You Slut-Shamed Me!

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…