This is not qualified medical advice, and does not apply to restricted diets.
When you have lost so much fluid from your system through diarrhoea you haven't got a lot of water left to give to urine.
Try and drink a pint of water every hour (hot, warm or cold) with a pinch of salt and a 1/4 teaspoon of sugar and squeeze of lemon juice. You can do the same with flat diet coke or full fat coca cola. You will feel better soon, but if after say 72hrs of the extra water / fluid treatment and you are still not weeing go back and get some medical advice.