I thought we were friends so why?

I have been best friends with this girl since last year. We understand each other and laugh at the same things. Now it's another school year and things started off really well...and then its like she stopped understanding me. I'm a christian and she doesnt really have a religion and thats never gotten between us. Idk what the problem is. Im naturally perky and very funny and NEVER the annoying type. I know when to stop and i know how it feels to be annoyed. Im a good friend. No there's this other girl we've both gotten to know in some of our cles. She is funny like me and I'm friends with her but lately it seems my bff has been laughin at her more and leavin me out? I'm sad. I don't feel that connection anymore. It makes me a little mad at the other girl. I haven't changed at all and things are hard at home. Now that i need her to support me she's not close anymore. Doesnt pay attention to me much. Whats happened to my friend? or is it just me?