With Molly I often have a similar reaction; it's rather interesting, as many report that ecstasy or MDMA hinders urination, but instead it increases it for me. Taking a sip of water every couple of minutes isn't going to result in overhydration, however. As long as you're not drinking half a bottle of water every 5 minutes, you should be safe. I think the reason it happens to me is that I'm usually pretty dehydrated on a day-to-day basis, and rolling just makes me a more aware of/less able to ignore my body's needs. Perhaps it is the same with you.
While frequent urination can be somewhat annoying, it's also not harmful. As long as you're urinating, you're probably further ensured that you're not overhydrating. And perhaps you feel less time has passed than actually has...a theory I have for my experiences with the same, but I'm always too engrossed in the roll to truly look at the clock for affirmation. ^^;;