Alcohol can make you feel warmer than you already are.
If you have a few drinks and suddenly feel toasty, you're not imagining it. Alcohol widens and dilates your blood vessels, making it easier for body heat to rise to the skins' surface. You feel warmer because you have a higher volume of blood that's coming up to the surface of the skin. When it's warm and sunny outside, you are at a higher risk of overheating.

Alcohol dehydrates you.
Being outdoors in the sun, you're probably already in an environment where you're low on water. Alcohol is a diuretic and strips your body of fluids. It causes the kidneys to release more water through urine than we normally would, making you lose water through sweat from the heat and urine. This can make you dehydrated and unable to cool down. It's important to drink more water during warm, summer days to avoid dehydration.

The good news is there are safe ways to drink in the heat.
You can have a beer (or two) without paying for it later by developing a few healthy habits. First, drink way more water than you think you need. You likely won't feel thirsty, but having a glass of water every 30 minutes will help replenish any fluids lost. Another way to hold onto extra fluid is to eat salt-based foods, such as tortilla chips. If you are playing sports, you should drink a sports drink that has a large amount of carbohydrates in it, because you lose those when you exercise.