Kim Kardashian Serves Kris Humphries' Ex-GF with Subpoena in Divorce

Kim Kardashian has just served Kris Humphries' ex-girlfriend, Myla Sinanaj, with a subpoena in the divorce case ... TMZ has learned.

As TMZ first reported, Myla was in a boyfriend/girlfriend relationship with Kris since mid-January -- something Kris was desperately trying to hide ... to the point his people lied about the relationship.

As for why Kris wanted to cover it up ... sources tell us Kris and his lawyer were worried it would have an impact on the divorce case. We know Kris demanded $7 million from Kim, claiming she devastated him by filing divorce papers. It's hard to say he was devastated when he moved on two months after divorce docs were filed.

Sources say Kim's lawyer wants to know if Kris said anything about his marriage which might relate to his claim Kim defrauded him -- in other words, did he blow his cover during pillow talk. We're told Kim's lawyer also wants details about hundreds of texts and emails exchanged between Myla and Kris.

Myla's lawyer, Joe Tacopina, accepted service on behalf of Myla.

Tacopina tells TMZ his client will comply with the subpoena.