Why does my girlfriend get angry at me when I bring up an app that I have used with my ex before?

We are in a long-distance relationship. I suggested during the night we can watch a movie together on Netflix. She didn't know how, so I suggested we use an app called Teleparty. It allowed us to watch together. She asked how do I know and I told her the truth, and now she is angry and declined my request. I am confused, it's just an application. If another application does the same function, I would gladly use it but it's too late now. I can't lie to her because she will eventually find out about it and it will sting more later. I dont even know why am I asking this question also...

Why does my girlfriend get angry at me when I bring up an app that I have used with my ex before?
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