A lady in our group had this problem early on, though I think she found it settled down after a few weeks.
A lot of the weight you lose during the early part of a diet is water weight.
From my non-scientific background I understand it to be because of the following:
- Even just drinking 1.5l, you are likely to have upped your water content greatly as fruit and veg contain a LOT of water. So not only does that have to come out, but your body is no longer starved for water and feels more relaxed about releasing the water it had been holding onto before as bloating.
- Water retention is worse the more fat cells we have (fat cells contain water too I believe). So as you lose weight, your body is retaining less water. That has to go somewhere!
- As someone above mentioned, many vegetables are diuretics (I know leeks are one).
It must be a real pain, but remember you are losing water weight as you pee it out! If it continues though do see your doctor as excessive thirst/urination can be a sign of diabetes or other blood sugar issues.