Carsickness on City bus - out of the blue?!

Although I tend to get carsick on long rides (symptoms include headache, generally not feeling well, but little nausea and no vomiting), I have never had a problem when riding on a bus (school bus or city bus). The past 3 mornings, I have gotten carsick on the bus! The first day I broke out in a sweat and felt like I might pass out. The second & third day weren't nearly that bad, but still uncomfortable. I have had no changes in my morning eating habits. Usually I am mostly fine by the time I get to work, and a Sprite seems to help. My (city) bus ride lasts about 45 minutes and my sickness has kicked in after about 20 minutes. Any suggestions?