Why is my ex-gf mad/jealous?

So I had a dinner with friends last Saturday, and I invited another girl. My ex learned about it and was apparently very upset, mad. Later during the dinner I received a message that the other girl isn't coming.
So I told one of my friend at the dinner (the one that told me that my ex was mad) to text my ex that the girl isn't coming. She replied to our common friend at the dinner "I love you for telling me that".
I'm not an expert, but that seems like jealousy.
Now today I asked my ex, by text, to finally give me my apartment's key back, that I don't feel good with her having it especially if she gets a boyfriend... and she just answered "K".
I'm not an expert, but that seems a little upset. It's worth mentioning that the text before I asked for my key was a normal full sentence.

My question is, why the f... is a ex jealous/upset, while she is the one that broke up? Why does she always criticize me when we see each other, but then is jealous if I see another girl?
Why is my ex-gf mad/jealous?
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