I seem to have a weird but inconsistent form of motion sickness. Let me explain.
Well my motion sickness started back in 2005 after the Tomato Festival was over. I had ridden a fair ride called Hurricane about 10 times in a row (the fair I went to before that, I actually got sick on a ride for the very first time). After the last ride I felt horrible and couldn't do anything for a couple hours.
Next year I go to the I-X Indoor Amusement Park. I go on Hurricane again but this time I don't feel that bad. I go on Round-Up and still don't feel bad. I ride Thunder Bolt however (it's a Matterhorn-type ride) and feel like I'm gonna throw up. But the funny thing is later on that day I rode a ride called Quasar which spins exceedingly fast and swivels up and down and nothing happened. Not even faint dizziness.
Then in '07 at Busch Gardens I rode Battering Ram, a pirate ship ride, and feel HORRIBLE after getting off. I didn't get sick though. I think maybe my feeling on it could be attributed to having drank coffee before getting on it.
This year we went to Cedar Point and I was able to ride Calypso and Scrambler fine without feeling too bad (only had slight dizziness for a few seconds). I was gonna test my luck on the Super Himalaya but chickened out.
I haven't tried any coasters since that incident that happened in '05 either, but I do know I can ride certain rides that go upside down that aren't coasters. I can't seem to marathon any rides anymore either.
Before anyone asks, I'm only 21 years old, so I don't think it's an age issue. My dad's triple my age and when he was slightly younger, like in his 50s, he was able to ride nearly anything and not get sick.