I'm sorry if it's inappropriate, that phrase I heard while I was watching American Horror Stories, but I don't think it has something to do with having s3x. In order to avoid something vulgar around here, I've changed the conversation a bit so that it won't sound inappropriate.
Anyway here's the conversation:
X: Do you really love me or are you just experimenting with love or are you just fucking with me?
Y: (Y grabs X's hands) No. I'm not fucking with you. But, I'm not sure with the other two.
I did try to search that phrase. However, sadly. Google let me down and gave me the inappropriate results. If I'm guessing it correctly, does it exactly mean like the phrasal verb mess with somebody?