I will get a headache if i get tickled for a long time. Not any nausea though. The headache is just from laughing and not breathing though. And it only hits if i sit up. So after a prolonged tickle session i have to lay down for about 15 mins...
Both are possible. I've known of several people who've experienced nausea, myself included. That has generally been from too much rough stuff and/or too soon after eating. Hyperventilation and anoxia can both cause one to pass out, as can dehydration, overheating and sheer exhaustion.
This is why people need to pay attention to more than the ticklish responses. After all, if your lee passes out, the fun is over. Who wants that?!
I had a friend who would start coughing and said she was nauseous if anyone tickled her, but I never saw her throw up, although she did get close a few times. I think it was just that tickling really distressed her and freaked her out.
I almost got sick once... we were on the Witches' Wheel at Cedar Point amusement park. I was sitting in front of him between his legs and he was behind me grabbin at my sides and goin up my shirt. I was helpless... and that damn ride took FOREVER to stop... almost lost it all over both of us. But he apologized and of course I forgave him. He's such a sweetheart!