4 Days Back, When I Was Doing Some Window Shopping

4 days back, when i was doing some window shopping in high temperature, suddenly i got mild feeling of vomitting, followed by dizziness and then suddenly i fell down and lost conciousness for few minutes. Thereafter i am normal. I am 43 and had no previous medical problem. I have bit of spondilysis..both cercical and low back..thats it.

Thereafter i am normal but feeling bit weak.

What can be cause of fainting? I have no habit of alcohol or smoking. I am slightly over weight but bp , sugar, cholesterol etc all normal