There is something really weird with motion sickness. In my case, it seems that it starts affecting me when I start thinking about it.
Example: Few weeks ago, my girlfriend and I were near Cancun riding a catamaran in long rolling 4-5 feet wave and we were having a blast (at least I was loll ). Everything was going fine until at some point I told myself “mmm … weird that none of us felt sick yet considering the sea condition”. Then guess what, as soon as I said that to myself, I started to feel a little sick. Not much … but enough to realize that, for me, it seems related to the fact that I start thinking about it. Not sure if it makes sense or not, but it happened to me more than once.