Is it normal to be jealous of my bff's other friends/girlfriend

My bff is essentially the only friend I have. We are extremely close and usually talk at least once a day. We always think of each other first when planning something (i.e. If spending money, we wonder if we should really buy it because we might not have enough left to see each other or spend on each other). We act a lot like a couple but we really aren't and neither of us wants to be. Lately I've been really jealous of his other friends and this girl he thinks he's falling in love with. I feel like they are going to take him away from me and we will never be friends again. I know it's stupid but I really do. Sometimes I'll check out his Facebook page and check out what they've been saying to him, even though I know it'll just make me upset. I feel left out when he talks about them or goes to visit them. I hate that they know a different side of him and that they might know things about him I don't. Is this normal or am I a psycho?