Is it okay to have sex with another girl two weeks after your girl broke up with you?

My girlfriend of 1 year broke up with me 3 weeks ago for another guy. Two weeks after she broke up, I had sex with a girl I've knew for a few years. I never cheated on my girl while we were together, and only occasionaly talked to this girl. Now my girlfriend told me she wants me back because she thinks it was wrong leaving me for another guy, she just wanted to see what it would be like dating someone else. But before I took her back I told her what had happened. She didn't have sex with this other guy, they only kissed and hugged, and I'm 100% sure it's true what she says. But now she's all mad at me and doesn't want to talk to me and stuff because I already had sex with someone else. All I want to know is, was it wrong to have sex so fast after a breakup? Even tho she left me for another man?