Earbuds & Motion Sickness?

I'm a Metro commuter (DC version of subway) and today, for the first time, used my Etymotics ER-6 earbuds with my iPod. Also for the first time, ended up with motion sickness discomfort... I've heard motion sickness is a result of inner ear balancing or something and wondered if my experience was the result of using earbuds. Anyone had a similar experience? I'm not generally prone to this kind of thing and I never had issues when using the ear phones that just sort of sit in your ear... (The etymotics actually kind of plug into your outer ear canal creating somewhat of a seal). I'm not anxious to recreate the experience for test purposes.