Hi
Welcome to the MedHelp forum!
The diarrhea could be due to something you ate even a day or two before. It is unlikely to be due to sitting in the sun. Take an OTC lactobacillus preparation and yoghurt with your meals. Drink plenty of fluids. Eat light, do not stay hungry but eat simple things like toast, porridge, cereals, crackers etc. Take ORS. Take over the counter antacid and antiemetic (this SOS for vomiting). Watch out for a day or two with this treatment. Take care!
Even if you are accustomed to eating oatmeal every day, it's still possible that it could be the culprit of your diarrhea. It may also be due to something that you ate yesterday or even the day before - sometimes our stools don't get affected immediately from what we've eaten.
Some medications can cause very adverse reactions to sun/heat, even with short exposures, so if you're taking any medications, they may also be the culprit.
Did/do you have any other symptoms, such as dizziness, headache, nausea?
Your diarrhea is likely due to another cause. If you have no rash or skin problems, "sun poisoning" is not your problem.