Tilted bus makes me dizzy, not moving!?

On my school sport days, I have to go on a bus to play against another school. The area my school is in has loads of tilted roads, as in, ones that slant down, and also ones that tilt sideways e.g. the edges of the road are slanting downwards. Every time I enter the bus, as soon as I step into the bus, I get dizzy and disorientated. As soon as i walk down the bus aisle everything goes weird when I see it's tilted. I don't really have motion sickness in boats or cars, unless I try to read. When I enter a bus tilted sideways, I actually get a headache and feel nauseated. Not the vomiting type of nausea, but just the uncomfortable, head-achey dizzy nausea. Why does this happen, and what can I do to fix this? Apart from drugs.