How long do you have to be in the sun/heat to get heatstroke?

The weirdest thing happened to me today. I rode my bike for about five to ten minutes through neighborhood streets on the way to a friend’s house. When I got there, and was standing at the door, I got an extreme “headrush” feeling. The light swelled up until I could only see basic dark shapes surrounded by light, a loud ringing was in my ears, I was dizzy, my legs were weak/trembling, and I felt nauseated. I sat down and waited for it to pass, which took about three minutes. I fully expected to faint or black out – that’s what I imagine it would feel like – but I didn’t.

It was hot today, probably about ninety degrees, but I really didn’t exhert myself that much and while I was riding I felt totally fine. Was this just a little heat exhaustion/possibly dehydration or perhaps something more serious?