My stomach hurts on rides that drop. Why?

I live in Australia, so we only have a few theme parks, and none of them are in the state I live in, but I have been to them 3 times in my life (which for most Australians is lucky).

The last time I went to one, I noticed that when I go on rollercoasters, my stomach really hurts when I go down the drops. It never used to be like that, when I was a child I was a complete adrenaline junkie and the first time I went to a theme park I went on the giant drop 3 times in a row (Drop tower ride). But now that I've gotten older, that butterfly feeling you get when you drop has developed into something a lot more painful. The last time I went on a rollercoaster was a few months ago, and it was the Arkham Asylum Escape (which used to be called the Lethal Weapon Ride) in Movie World, Queensland, which used to be my favourite ride when I was a kid. I would ride it over and over again as a kid, but this time when I went on it, going down the drop hurt so much that I had to shrink down in my seat and apparently I let out a strange gurgling/grunting noise.

I'm not afraid of rollercoasters at all so I don't think it's psychological. I have no fear of heights, or going through the loops, nothing like that. Infant, I love rollercoasters and if I could avoid going down the drops, I'd be on them all the time.

At some point I'd like to visit some american theme parks, so does anyone know why my body does this? And if so how do I stop it?