I definitely recomend going back and getting sunscreen. This is a reminder and a how to guide. And its very simple.
If you are in the sun right now
Do whatever you can to get out of the sun. Find shade, pour water on exposed skin, cover up.
When you get out of the sun
Do whatever you can right when you get out of the sun. Use aloe, drink cool water, rest and go easy and stay entirely out of the sun.
Sunburn and Sun Poisoning are both not very fun. I can tell you. But lets just avoid it entirely.