When I was 18-19, I was vising my girlfriend while a friend of hers was home from uni. My gf had me restrained to her bedframe (and, yes, the friend was well aware of our antics) and was tickling me. My gf invited the friend to help. The friend ended up being a far more fiendish and creative tickler than the gf. She had me howling like a wolf, and the two of them tickled me senseless. I'd never been tortured like that before, and I felt myself becoming light-headed and experiencing vertigo. Once I conveyed my issues to them, they let up. I was dizzy and everything seemed out of focus for most of the day after that, but it was so euphoric at the same time.