Sexual coercion is a crime. Report her to the cops.

Or, hold your head high. If someone "accuses" you of having sex with a fat chick (no crime, there, IDK why you had sex with her if you were going to be an A-hole about it), either say "My smokin-hot sex life is private, and I won't confirm or deny any rumors, except the rumor about how 'Sexual Coercion Girl's Name' literally tried to blackmail me into fucking her. I haven't decided if she did that because she's desperate, or she's a sexual predator who belongs in jail, or both." Or if the "fat chick" doesn't mind people knowing, say "Yeah, it it was AWESOME! More cushion for the pushin' if ya know what I mean."

Going forward, don't fuck people you don't want to bother to be decent to, to speak respectfully to and about them. And be careful of what you message or post, even when you think it's private, because apparently you were so ashamed of having sex with an overweight person, yet somehow your "enemy" found out about it, so how did that happen? If you want your sex life to be private, don't post/message/send anything you wouldn't want everyone to find out about.