I did my long run on Friday at 10am so the sun was already well up. It was our first proper "hot" day of the year at 23c (73f) - know that's not hot for some people but its been 2c-8c (35f-46f) for the past 5 months. I was also in direct sun for most of the run, very little shade. Since then I've felt absolutely trash, had a terrible run yesterday and today still didn't feel great. Have felt slightly short of breath too.
Usually I run before work on weekdays, the week before it was ~2c (35f) most mornings, and my last long run it was raining and cool, so less than 8c (46f) too. Pretty much every long run or session for the past 5 months I've worn gloves and a long sleeve, on Friday it was split shorts and a vest.
Did I just do a real dumbdumb move doing a long run on the first proper warm day and really cooked myself? I probably did but it's hard to take a step back when its happening to you - have spent the past 2 days wondering if something is seriously medically wrong.