Hillary is down with some sort of health problem, possibly heat stroke. More information as it comes out and someone else posts it, because I'm far to lazy to chase up links.
She doesn't sweat, and it was a hot day there. The story here is "Hillary stayed in the heat for longer than was wise," possibly because she didn't want to be seen leaving early. Although what she was wearing didn't help. From personal experience, the less you have on and the thinner it is, the better, because the only way you can get rid of heat (besides artificially cooling yourself in some way) is to radiate it from your skin, and clothing acts as insulation, and black clothing is pretty much the worst because of how well it absorbs sunlight. However, she was probably wearing what she was wearing because of the nature of the event (it being about 9/11, so black clothing is the most culturally appropriate thing), so...
I don't know if she can tell if she's getting too hot before the effects of heat exhaustion etc hit her. Personally I start to feel like I'm itching all over, and by itching I mean it feels like mosquito bites, but I've never been able to find anything on the Internets to explain that, and my doctor was baffled too.
P.S. Advantages of this kind of condition include never getting sweaty palms.
P.P.S. This is exactly right:
http://www.dailykos.com/story/2016/09/11/1568977/-Don-t-panic-Hillary-Clinton-just-doesn-t-sweatP.P.P.S. Orange Wizard got banned? Oy vey.