What Caused This Extreme Light Headed Feeling On Ferry?

I went over the North sea on a ferry and it was an awful experience. Captain said it was a force 9 gale and the boat was rocking everywhere. I became extremely light headed and dizzy. I didn’t have sickness or nausea, just this awful head problem.
What was it? I am going on a cruise to Canaries in October and am worried it will happen again and ruin the trip.