My girlfriends best friend flirts with me (I think). Should I tell my girlfriend?

Anonymous
I am not sure if she flirts with me on purpose but it certainly seems like it. She looks intensely into my eyes a lot, when we're alone she changes the subject to something sexual. Yesterday was especially weird. My girlfriend still lives with her parents, in their basement. All three of us were down there and my girlfriend went upstairs to get drinks. We were talking and she turned to look at me and in doing so put her hand on my thigh. So we were just looking directly into each other's eyes with her hand lightly on my thigh talking about nothing. I started to get hard (I can't control it) and it was becoming increasingly uncomfortable so I got up to check on the drinks. As I did she looked at my crotch and gave a girly "ooooh" sound.

Then when we came back everything was normal. Am I being crazy? Girls never show interest in me like that, is she just acting normal? Should I tell my girlfriend? I'm kind of confused.
My girlfriends best friend flirts with me (I think). Should I tell my girlfriend?
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