Are my friends deliberately leaving me out?

I have a big circle of friends in school. I'm 17, and I've always had 2 or 3 best friends, but lately, I've noticed that I'm being left out. A couple of months ago, my good friend that I could talk to about anything randomly stopped talking to me, and discussed her problems with my other friends. She never tells me anything anymore, and recently, she was sent to hospital. I won't discuss why, but not one of my friends told me about it, and so I felt bad because they'd all been to visit her, and I hadn't. It wasn't until she came out of the hospital that I found out. Also, the four of them always go on shopping trips together as friends, but I'm never invited or told. I talk to them all regularly, and I consider them my best friends, but they seem to be leaving me out of stuff. My other friend is also in hospital, and nobody ever invites me to go and see her. I don't know what it is I've done that's caused them to leave me out of things, so should I talk to some of my other close friends in the group? I'm also feeling very depressive, but my friends seem to think I'm not being serious, and I don't feel I can talk to them about anything anymore.