Heat rash causes small red bumps, mild itching, and prickly skin, usually in sweaty areas like the neck and chest, due to blocked sweat glands from heat and sweating. Sun poisoning is a severe sunburn caused by prolonged UV exposure, leading to intense redness, pain, swelling, blistering, and sometimes fever or nausea, mainly on exposed skin. Heat rash is mild and sweat-related, while sun poisoning is more serious and inflammatory. You should see a doctor if there is severe pain, fever, large blisters, swelling, or signs of infection.