Is it normal that i'm hurt over what my best friend did?

I used to date a guy for a while. Him and I were very close and very much in love, until some issues came up and I broke up with him. Everything was fine, until he decided to ask my best friend out a short time after that. I didn't know about this until after they already begun going out. I must admit, I was a little upset, but I kept quiet, even though I did tell her a little how I felt. A month later, he breaks up with her, saying that he's still in love with me. Anyways, two years or so pass, and during that time, (without me saying ANYTHING), my friend runs up to me and apologizes constantly and says she felt really bad about the whole thing. She kept going on about how wrong it was. I explained to her exactly how I felt, and we moved past it. Well, just this morning, I call my friend at noon, and she sounded very tired and said "I was still sleeping. I Had an all nighter the other night.. obviously, I told you that." I asked her why she had an all nighter, and she said she was hanging out with my ex just "as friends" at a drive in movie marathon. I was a little surprised, since I thought neither of us were in contact with him anymore, but it sounded like a date, so I jokingly asked if he tried to kiss her. She said they hugged for a "very long" time, then he kissed her. For some reason, my heart sank. I am very upset. Is that normal?
I mean, I'm really mostly upset that she knew how I felt about her dating him, how she apologized constantly, and then she turns around and does that. And I know her, if I confront her, she'll probably say HE kissed HER and there was nothing she could do about it. And that they're just friends, even though she never told him that.
I'm just really confused.