When you say 'loads of water', how much is loads. If you eat lots of fibre then you must drink at the very least 2 litres a day. If that's not working, then even more and also increase exercise. If laxatives are not an option (which I agree has to be a last resort anyway) then the answers so far are really your only option.
If it was going to be a laxative though, I would suggest Movicol instead of the type that just purge you.
Hope something works for you soon - it's a horrible feeling and starts to become the only thing you think about!