Why is my gf being such a witch?

Well, recently my girlfriend has been acting much more angry towards me. Like this anger just spurs from nowhere and I'm not sure why. Also, sometimes when we talk I know she is doing other things like being on the internet and stuff like that. Then when I confront her about it she just gets mad and yells at me and tells me to stop keeping tabs on her. Before we used to talk all the time without interruption. How do I talk to her about this without her going off?