How to tell when your friend I shunning you?

Kso my best friend(lets call her jess) who also has another bestfriend(monica) and the 2 of them have been friends waaaay longer than me and jess have been friends with so I expected them to be a little closer but latley me and jess are barley talking and when we walk home she doesn't talk its just me and monica who I am now pretty good friends with. Like me and jess used to be super tight (she said she told me more than anyone because I related better, and we spent basically he whole summer together without monica) and she always told me that but latley she's been spending more time with the other girl and not talking to me as much. This could be a thing where monica said that they have barley been spending time together so now they are spending more time together to even it out but I don't know. This doesn't seem like jess and latley I've kind of been shunning her too because I'm not going to make an effort if she doesn't. But the thing is when ts just me and jess we have a great time but when its all 3 of us she's completley different. So basically we were really close and now not as much just wanted to know if that sounded like something to you guys or is it just me. Thanks