If your hot drink is tea or coffee it will have caffeine in it (unless it's decaff).
Caffeine is, among other things, a diuretic -- it stimulates the kidneys to increase the amount of urine they produce, and eventually this has to go somewhere.
Other diuretics include alcohol, dandelion (which is why the French name is pis*-en-lit: wet-the-bed) and of course there are drugs specially designed to do it for medical reasons.
I think chocolate has a little caffeine in too.