What should I do with my sensitive girl friend?

Ok tonight I made an ex joke to my girl out of nowhere. So my girl's ex's name is "Eden". I made fun of it " Garden of Eden" if you're familiar of it it's in the Bible. The name is just too funny for a dude to not make fun of. Also, he was a douche for cheating on her with her bestfriend and choosing her bestfriend over her cause hes more into the bestfriend and i dont get the **** why he didnt date the bestfriend in the first place if he's more into the bestfriend. It irritates me to think about it, that he did hurt my girl that way, cause my girl admitted to me that she hurt her feelungs for that douche and cried while drinking vodka overnight i was there cause she was talking to me while she was drunk not personally but via text. But still, you can feel her pain that she was hurting cause she was saying that she really liked that douchebag and i don't understand why some women would date that kind of men, that is you know, shitheads. I don't know why they can't see through that some men are just full of **** and they deserve so much better than to be treated like crap. I really like this girl tho, we're not official or anything but she is very special to me and she said I am too for her. So ya, I joke about it sometimes but tonight she just got mad at me for it, calling me annoyingly immature because of it. Cause i was bringing up the past and making a joke out of it. I just don't understand cause I thought they weren't that serious cause she told me so, that they just dated for a couple of months so they weren't official or serious at all. But I don't get why she was suddenly mad at me about it, about a stupid ex joke. So I said:

Me: "I'd rather go in a garden this valentines day"
Her: "Ohhhh garden agaiiiiiiiiiiiiin, idk if ur obsessed wth garden or what"
Me: "Ya I am, garden of weed, is that bad?"
Her: "Oh im obsessed with u tho, is that bad?"
Me: "Like when u were obsessed with a garden bfore? Orrr?"
Her: "im never obsessed wt a garden, why are u making up stories?"
Me: "oh really, I thought a garden hurt ur feelungs once, ahhh 8("
Her: "ok i dont want to talk to u anymore, ur annoying pfft"
Me: " ok my bad, told u i was annoying"
Her: " you are and sometimes u dont know what ur saying anymore, cause its not funny"

And so on, ofcourse i wanted to explain my side so i kept replying to her arguments or what she says and she got mad about it like she told me: " i don't want to talk to you today."
So i don't know what should I do? Why is she acting like that? Why does it sound like she is hurting still? Cause I thought exes or past arent suppose to bother u anymore unless there is still something there. And yes, I did apologize right after she got mad. Help.