My boyfriend of 5 years put a girls # under the initail K and lied to me and said it was a guy "Big K"?

Unfortunately if he's lying to you about it then it's because he's doing something wrong or thinking about doing something wrong. Talk to him and tell him to act like a man and tell you who this K is and to tell you the truth. Don't let this go until you find out the answer because if you turn your cheek now then it will continue happening.