Why does my "friend" act like this?

So my friend and I hang out sometimes and i feel that she treats me in a bad way. Sometimes she asks me "wanna go out on friday?" and the we make plans to go out and then when friday comes she ignore my texts and calls or says that she can't because she has to do something but then i find out that she went out with someone else. This has happened like 4 times and everytime it happens she calls me like the next day and pretend like nothing, and I ask "why didn't you call" and she is like "oh I forgot!" If she doesn't want to go out with me why even ask or why can't all three go out?

Sometimes she also asks if I want to go shopping or something and then she goes with someone else and ignores my texts. And she is the one who asks me if i want to go out? it would be another thing if I was the one asking and she had problems with saying no? I don't understand why me because I've done nothing to her.

For a couple of months ago she started liking ALL of my boyfriends pictures. She knew it bothered be but she didn't stop?then I asked him to unfollow her so he did.

Yesterday I went out with her and It was the first time in months she didnt do what she use to do.. She took a picture of her and her friend and posted it on ig. I commented it with something nice and she answered on the picture with something nice back and today when I looked at the picture she deleted the comments? Why is she like this to me?