I have read a lot of posts today from disappointed people. Guys it is very hot and humid out there! This affects your body (and mind too probably). I waited until this evening to run (amazingly did it btw) and still got cramps on a short walk to shops later.
Someone posted a link to heat exposure earlier. I would say even for short runs take water, not just to drink but it is good for "additional sweat" to cool you down. Go for the drowned rat look :). As always, listen to your body, collapsing from heat exhaustion will not help you progress.
btw the is NOT an excuse not to run, just try to adapt a bit and don't be too hard on yourself.