Everytime I'm Outside Doing Yard Work, It Can Be For

Everytime I'm outside doing yard work, it can be for as little as 15 or 20 minutes, and I get really exhausted and everything turns black. I start breathing really heavy. I thought this has something to do with the heat, but I can be inside my house in a air conditioned house and doing house work. I don't start breathing heavy, but when I bend down and come back up, everything goes black. I have mentioned this to my doctor a couple of times, but she gives me no explanation for this. I know this is not normal. This happened again thus weekend and I had to go in my house and recoup for about 20 minutes before I can go outside again. I can sit outside just doing nothing and nothing happens. This only happens when I'm pulling weeds etc.