Is it normal to feel anxious about this?

I've been dating this guy for about almost a year now, and this month I decided to join his group of Facebook friends- he didn't tell me about them until they were practically harassing him to upload a photo of me because apparently this was a huge deal since he's like the youngster of the group.

He showed me his one friend, (I'll use a different name) Kat, and in a call afterwards, he'd gone on to a topic about how he used to have feelings for her, how she's super pretty, and kind, etc.

Now, I didn't have an issue with this at first - considering the fact that he USED to have feelings. But, that literally went away as soon as he would keep on bringing her up in our conversations for a whole week. It made me feel like he still had feelings for her. But he'd assure me that he only had his eyes on me.

The day after that incident, he told me in another call that he was in the park with Kat and one of her friends while playing Pokemon Go, and Kat started asking him about my age (and possibly some other stuff, he didn't say anything else), when he told her how old I was, he said she started to act weird (I'm older than him by a year, and turns out Kat is a year younger than her boyfriend - Kat and I are the same age, except she's older by a few months). He said he felt weird, like she had feelings for him.

It went to the point that I'd start panicking and I started to break down crying because he made it feel like he was second-guessing who he REALLY wanted to be with. Like I had to be scared that he'd pick her over me since we don't live that close to each other.

Plus, he admitted to having feelings for her before I started going out with him. So that just added on to my anxiety.

When I started crying over the call, I think I made him panic a little because he hates it when I cry. He kept apologizing for hurting me, then he started to cry, too, saying how he loves me, that he wouldn't choose her over me, that he'd stay with me no matter how attractive or kind she was.

He also said stuff like "I stayed with you for this long, that should mean something. You are a partner unlike any other I've had."

It calmed me down, but I still can't look at his friend the same way now. She kind of bothers me with her presence online and I don't know why.
I mean, she's really nice.

Is it normal to feel this stressed out over this kind of situation? I feel like I'm really over-reacting.. I feel like a horrible girlfriend for being like this, to be honest. I'm a horrible person. :x