Why do I feel sick? Been feeling sick since last night.?

I feel really sick. I've felt this way since last night. I've had a nice shower to cool down. And that make me feel a little bit better last night. But it didn't make me feel any better tonight. After my shower I got a very nice cold drink of water and put it in a bottle, so I can drink it in the night. I got all cosy and warm in bed. So I'm all nice and relaxed. I've told my mum I feel sick, but she just thinks I'm trying to stay off school.