Catrin- Glad you got over that nausea soon after getting inside and getting your core cooled. I agree with the others, pre-hydrading and re-hydrating will help a lot. Your body needs the proper fluids and electrolytes to regulate your core temp.

I used to get bad cramps and nausea minutes into strenuous rides in the summer. Thought it was me just being soooo out of shape. I ended up cutting the coffee to one cup in like a month or more, and replacing it with not-caffeinated tea that I drink throughout the day if at work to re-hydrate for the commute home. Once the tea was finished (I used a French press at my desk with 4 red or camomile tea bags and a little honey) I switched to ice water. A few Bloks or similar electrolite chews just before the ride and half way home, and no cramps or nausea.

It's not just a pre ride thing. Most people are dehydraded as a constant state. It takes a conscious effort to hydrate properly but I feel better overall if I keep the fluid intake UP.