Why does my girlfriend do this?

I have been with my girlfriend for like 4 months now and she stays at my house a lot for sleepovers. But for the last month or so, she has been doing this annoying thing where she wakes me up by doing something annoying/painful/gross. She poured milk on my face. MILK! On the bed! Another morning she slapped me in the face and as I opened my eyes she had her butt in my face. She has done even worse than that to. But why does she find that funny? I've told her to stop but she hasn't.

Why does my girlfriend do this?
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