Heat sensitive/sick feeling while in heat?

I've had this problem all my life.

If I'm out in the heat for a while i feel horrible. Like I'm about to get sick, lightheaded, heavy sweating, etc.

I stay hydrated, I'll drink a bottle of water before hand if I know I'll be outside, and keep water with me.

I think it's connected to sugar because once I'll drink a sprite and feel better, and once my friend was with me and it happened and we went into this coffee shop and she got cheese cake. I tried a bite and it made me feel better.

But, I'm their is a fix, because this summer I'm going Costa Rica and don't want to be worrying about whether or not I'll feel sick. I can't even continue walking when I start to feel ill.

Is there a anything this could be?

*Note* I'm very small. This has been going on for all my life, like I said.