Dizzy after exercising?

So, I ride my BMX frequently and I do it vigorously for maybe a little less then a mile. And when I get home and get something to drink I sit down and I get really dizzy feeling and my vision is kind of impaired. I recently stopped smoking and I have anxiety problems. This only started happening recently. I'm 16 Years Old. 6 foot tall and 190 pounds. And the doctors tell me I'm healthy.