If you haven't felt seasick before it seems strange that you would change. Sounds to me like your stomach was still delicate after the illness the week before, then you might have eaten a dodgy mussel - so your constitution was weakened and this made you susceptible to feeling seasick. Next time you'll probably be fine, but with seasickness I think a lot of it is in the mind. I always feel seasick but have never actually been sick, just get a terrible headache! I had really bad morning sickness when I was pregnant and tried ginger and the pressure wrist straps and anything else that was suggested - but none of them worked!