I go out to a friends house with my boyfriend every weekend, and I'm normally in the heat, but not direct sunlight, after being in the heat for about 10-20 minutes, I get really light headed, nauseas, and start seeing black blotchy spots in my vision and have to sit down immediately or I feel like I'm going to faint/pass out. What would cause this and what can I do to stop it?

You need to drink plenty of water. Not alcohol, not caffeine, and do not chew ice. Lots of water or Gatorade.