Why is my heart broken?

Helen texted me again today at work. It's really no new thing since we've still been keeping touch even long after I ended things. Sometimes I wonder why I did, but I keep coming to the answer that it was too risky since I never really had enough information to know that we were good together.

But back to the texting. We exchanged the typical pleasantries that were common;

"hey, how are you?"
"Good, good. How are things with you?"

ect ect.

I know I broke her heart when I ended it months ago and I could always detect in her tone when she was having a bad day. However, today she was rather chipper. She used the word "peachy" which caught my attention. When I asked her what was going on, she simply said,

"Not sure you want to know. Mostly I'm trying to enjoy myself."

I knew what that meant and somewhere deep inside, my heart sunk.

Yeah, she met someone online and has slept with him. Helen says that it means nothing and that she's not looking for a boyfriend, but the thought of her sleeping with another guy just made my blood boil.
Why? Why now?

I have already done this to Helen, so I was in NOOOO position to be angry with her and I really wasn't. I was jealous. Why should I care? I was the one that ended it. I was the one that broke Helen the news that I was seeing another girl. Why am I so bothered by this?

I don't have an answer today. Maybe I will tomorrow or next week.
I really hope this isn't true love and I'm just being a total idiot...