This probably happens from gravity affecting the flow of blood to your clitoris
(take this seriously, please). When you become aroused, and especially before orgasm, your clitoris becomes engorged with blood and basically becomes "erect" like a penis. Its possible that the centripidal force (if you dont know what that is, look it up) caused from swinging caused more blood to enter the vessels in you pelvic area than normal, which could cause your clitoris to become extra-sensitive to any touch whatsoever (e.g. your underwear rubbing up against it).
This may seem a bit far-fetched, but thats the only thing I could think of.
Oh, and don't worry, I know alot about this stuff (being a guy and all) because I want to be a doctor someday (so please don't take my explanations as 100% accurate, though I answer to the best of my knowledge; even with female questions)
I hope this helped a bit! Don't be afraid to ask about anything, I'll try to find the most logical answer to any concerns or curiosities!