Girl's Behavior

What does it mean if a female friend sits on your lap?

We were at an after Halloween party and she sat on my lap. She was wearing a mini skirt and after awhile of I became fully erect because my friend has a very nice body. Her legs got very warm and she eventually smiled and got up without saying anything. Am I reading too much in to it if she invited me over to watch a movie on the weekend?
What does it mean if a female friend sits on your lap?
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