My friends left his house for the weekend and my GF's sister wants to have sex with me?

Well My GF's Sister (Twin) came and some friends we had a sleep over and me and my GF and her twin was in there and i started kissing but i dont think it was my gf cuz when we had sex she was like STOP IT HURTS and I shoved it in and kept Thrusting and she put her legs around me the next day she winked at me and my GF said Why is There Blood on the Sheets i just said I dont know and i got mad and Cursed her sister out and she was like dont act like u didnt love it so i said Never ever look at me or my GF again you Evil ***** and i told my gf and she just hugged me and said i Understand its an Easy Mistake just we are never gonna see her Again was i wrong?