Too much caffeine
I went to an amazing som tam (papaya salad) restaurant for lunch to meet a Couchsurfer called Tanja for the first time. We chatted for a few hours and moved to a nearby cafe where I had this rather large amount of strong coffee. We were joined by Tanja's friend Lera.
We went to the huge Sunday market (which has grown since I was last here) but I started to feel ill, presumably from a mixture of too much heat (it's always around 38°C), dehydration and too much caffeine.