You could be dehydrated due to the increase in temperature outside.
Have you felt like this in past as well? Do you exercise regularly?
Next Steps
Drink enough of water and juices( not aerated drinks) . if still not better will need to see a Physician to examine you.
Health Tips
Drink at least 2.5 lits of water everyday.
Avoid the sun - use cap / umbrella
Wear light clothes.