Earlier today TMZ broke the story about Kim Kardashian being furious at 'In Touch' running a cover story claiming that she has been cheating on her fiance Kris Humphries with Bret Lockett, an NFL player who also adds fuel to the fire by saying he has PROOF. Kim is threatening to sue 'In Touch' magazine and Lockett, but is her fury a sign of guilt?

TMZ got their hands on a demand letter from Kim's lawyer

threatening "In Touch" that if it ran the cheating story Kim would take legal action.  The letter put "In Touch" on notice ... the cheating allegations "are, at a minimum, damaging, defamatory, and an invasion of her privacy.

Kim says that she has never even met Bret Lockett. Bret Lockett, on the other hand, says that not only has he met Kim, but can PROVE IT. He told 'In Touch'

"I knew this was a game to her, and this is what she does."  Lockett adds, "She pursued me."

Lockett says that Kim's legal threats don't scare him one bit because he can back it all up with phone records, texts and he claims to even have 'personal' cell phone pics of Kim -- and if he has to, he's willing and able to share it all in court.

This guy seems pretty dead set on having what he describes as CONCRETE PROOF that his story is true, so we must ask --- Do you think Kim cheated ???