I have been getting severe headaches for the last week and a bit now. what could it be?

I have been getting severe headaches for the last week and a bit now. I also get really dizzy when I get the headaches. I have been drinking plenty of water so I'm not dehydrated. I got my first severe headache (recently) last tuesday when i was walking back to school at about midday. i had a hat on , i was wearing sunscreen, and i was drinking water the entire way back. i was also wearing a shirt that protected me from the sun. i got the headache and then got so dizzy i thought i was going to fall over. everything went blury and i couldn't focus properly. i took some panadol and continued on my way. however, i have been getting headaches just as bad since that day and i had to take a day off sport last friday because i was feeling really bad (headaches and dizzy). i do lots of sport and drink plenty of water. it is hot where i live though, as i live in the tropics. i don't know what it is and i have'nt gone to the doctors. does anyone know what it might be?