***I think my best friend is a lier?*** :(?

Well he is one of my best friends.. I have caught him lying to me once long time ago, but it was something small so I let it go and never mentioned it to him. Then just last week there was a situation where 2 people were involved, him and this other girl. He told me his side of the story first, and I believed him and everything, but then when I told her his story she said hes lying and she told me her side..I was suspicious so I asked him about it, and he denied certain things, and kept changing the subject..I trust this girl very much and I know she wouldnt lie about something like this, but for some reason I didn't get the same vibes from him, something in me tells me he's lying, I dont understand it. Now every past occurance is making me wonder if he lied about all those things aswell and everytime he tells me stuff I think hes lying. My perception of him has changed this year and I hate it! Maybe I'm over thinking things but I dont know! He's one of my best friends but I feel like hes a lier, and thats all that comes into my head when I think of him; I think this is very unhealthy and I want to talk to him about it but don't know how to start..what if he gets mad at me and doesn't see me the same way anymore? I dont want to lose him as a friend, cause honestly he is one of the best..even if he might be a lier. :( What should I do??