It is often not realised that motion sickness doesn't actually need any motion. The eyes in perceiving motion can achieve the same result. I recall flying a simulator with no motion, I felt as sick as a dog afterwards. Head movement can also be one of the causes. Normally, people grow out of it and put it behind them. You can theorise as much as you like and take pills and potions but they seldom have any effect other than add to what you bring up. A full stomach helps.