Is it hard to trust your boyfriend or girlfriend?

Yes, I know we all truely love our bf/gf, but isn’t there times when you think somethings up? Like he/she talks about another person alot, that person always calls your bf/gf, and invites only them and not you? I feel that way… And I was wondering if any of you know how I feel and is it normal and if you had any experiences like me. Thanks :)