My Friends are Constantly Leaving me Out?

I just turned 13 on Sunday, and this has been going on forever now. Not since I've turned 13. You see, I'm friends with two girls(No names..sorry.) The one girl comes over my house sometimes, and the other just moved. I've never been over the one girl's house before.(We'll call her M, and the other will be K.) M said: Im gonna have you and K over for a sleepover very soon!! I was excited. I was letting go of the fact that she always had K over and never me. They always bragged about it on Facebook.. Anyways, I logged on today, and K started chatting with me. She said that she is going over M's house to swim and sleepover. I was so upset and still am! I've been crying all day! They BOTH bragged about it on Facebook! They expect me to just get over it and it's not that easy! I feel incredibly mad, sad, and just plain weird. Do they not like me? What's going on? Also, on top of that, a girl(we'll call her C) used to come to my house all the time. Now she thinks she's all grown up and never talks to me anymore. I'm somewhat related to her now, and she lives right next door, soooo I feel like I HAVE to invite her to my family functions. She's constantly putting stuff like: Im going to my friend's(not naming names for this) house! Cant wait!!!! :D and stuff life that. She NEVER puts pictures of me and her up on Facebook(I do sometimes but she rarely visits anymore.) and she NEVER mentions that she is doing something with me. I do every single time! I try to be nice but it fails. I have a bad headache from crying! I feel so left out. I've always been nice to everyone and a kind and caring person. I honestly need help. Bad. :( Please answer nicely, nothing harsh...thanks.. :'S