Idk what this feeling is?

Hi, so I’m just really curious on what this is and why it happens. I’m assigned female at birth, and go by she/her, and whenever this certain guy walks by me or is just around me, I get these like butterflies down there. Its so weird, and idk why it happens. Is this something both people feel? Because every time he’s around me my knees get weak and I just get that feeling. But I don’t ever think of him sexually at all. Sorry I’m just so confused as why it happens every time I’m around him LMFAO😭