BRAZEN PRANK! What should I have done?

My flatmate is sometimes really nasty and plays mean tricks on me all the time... This time she really took it too far, we were on a PUBLIC BEACH and she wrote "FUCK ME!" on my back with sun cream. What's that all about? Everyone was watching me and one of them asked me if the offer was serious... How was I supposed to react? Everyone was staring at me.