Why does being alone hurt?

I've been really upset lately because this situation has bothered me for a while. Some of my friends are ditching me at school. My best friend and one of my other friend have be hanging out with these other kids pretty much every day. My best friend has been acting pretty funny towards me lately in my opinion. She doesn't say "Hi!" to me anymore like she used to. It was her birthday two weeks ago and she gave cake to her other friends and didn't give me any. We've been best friends since we were babies and I hate when I get treated this way because I try to make her happy after all she's been through most of her life. At lunch today, My friend, Caesar and I sat lunch together and he watched and comfort me while I cried. My eyes are red, puffy, and swollen from crying my heart out because I feel like I'm always alone. She asked me what was wrong and told her "nothing". I was figuring out why do you care how I feel? She always think she doesn't do anything wrong and trying to act all innocent.Sometimes, I don't want to tell her how I feel, but I don't think she cares and I'm losing my trust with her. Everytime when my friends leave me by myself, mostly my best friend, I feel left out, alone, and hurt. What should I do to make myself happy?