How My Best Friend Stabbed Me In The Back

How do I even begin this story. Should I call it jealousy, Envy or is someone trying to cook up a 
story? You guys would help me judge better by the time you finish reading my story. 

I have a best friend who we sometimes refer to each other as brothers from another mother. My name is Chris and my friend's name is Chuks. We've been friends since childhood days. He's an extrovert while me on the other hand is an introvert. Despite the fact that we are different in so many ways. Yet We are best of friends.  Chuks my friend started getting intimate with women since we were in our final year in High-school. 

He tried so many times to hook me up with girls but I always turned him down. We finished high school and we were lucky to gain admission into the same University. I had my first girlfriend in 100level but ever since I met this babe, Chuks has never for once been in support of our relationship. Always saying bad things about her. It got so worse that my girlfriend later found out that Chuks does not like him. This is a woman whom I have a very promising relationship with. She's brilliant, beautiful, humble, kind, generous, homely. In fact, She's every man's dream. And yet my best friend has got a problem with her? 

One day, I decided to travel home for a few days. While I was away, Chuks went to visit my girlfriend and told her so many bad things about me. He said I've got so many girlfriends, that I even have a betrothed wife at home and She's the person I travelled home to see. He even promised my girlfriend that he was going to hook her up with a correct guy. That she should break up with me. 

My girlfriend promised him not to tell me anything and that she would take his advice. When I came back, she narrated everything to me but insisted not to tell my best friend. I promised her not to tell my friend about it but try to keep at arms length with him

Here is what is bothering me, I don't know how to pretend, I really feel like confronting him. I mean, he's supposed to be my best friend. Why on earth does he want to spoil my relationship? This is the very first relationship I've ever had and he has his own numerous girlfriends. So I can't say he's lonely or jealous.

What if my girlfriend is lying against him just to get back at him for not liking her? Have you guys thought of that too? Who do I believe in this kind of situation? What if I confront him and he denies it? I really need your advice? I don't want to handle this issue the wrong way. Who do I believe?

Please what is the best thing you suggest I do...?