Why Am I ALWAYS Second Choice to People?

I honestly feel like everyone is choosing someone over me. I'm a girl who has one main clique but I make friends VERY easily so I'm sort of just in a lot of sub-cliques. Anyways, all my friends ( and I literally mean ALL ) have someone that, if faced in a dire situation, they would save someone else over me.

My main clique includes Me, Aliyah, Hannah, and Taylor. So basically I used to just be the main focus of the group because Aliyah & Taylor used to argue a lot, so they just sort of relied on me to even things out. Then, I met Hannah and introduced her to the group. We've all been a group since AUGUST 2013, and I've noticed some exclusion of me along the way, but lately it's just been building up.

First, Hannah had a sleepover. She invited Aliyah, Taylor and Sam (another girl we all know) . You know who wasn't there? Me. I was a little offended but just brushed it off really. This exact scenario happened many, many, MANY times after that, but, again, I brushed it off. We all got assigned to the same art class back in January, and this is the jaw-dropping moment of which I realized how little I meant to them. We all shared a table in that class and we were just drawing when suddenly I realized Taylor wasn't at our table. Instead she was at the table behind ours. And she came over, whispered something to Aliyah and returned to her seat. Then Aliyah moved too. Then they both started calling for Hannah and she moved to. I was alone at the art table and none of them seemed sorry. AFTER TWO WHOLE DAYS they realized I wasn't talking to them so they came back asking for my forgiveness. I forgave them. But ever since then, I still realize they don't actually think I'm important!! They still exclude me from sleepovers, mall-meetups, after school hangouts, and group texts! They do not try to hide it by the way. They're completely oblivious to the fact that I'm even there and just accidentally flaunt it all the time.

A normal conversation between us all goes like this :

*me and Hannah/Aliyah/taylor talking*

*one/two of the others sits down*

Whichever one was with me first : Oh hey ______ are you coming over tomorrow?

________: YEAH! You and ____ (< not me) should come to mine next week

Me; *UTTERLY EXCLUDED*

Honestly it sucks, and I don't even know if I'm just over reacting? So any advice with dealing with this? Or am I just blowing this out of proportion?? Thanks.