Well in my opinion you should ask your doctor for help and follow his advice. It is great to be on a diet but if you do not feel well and suffer from excessive fluid it is a good time to ask your physician for advice.
The trouble with natural diuretic foods is that some of them have strange side effects and should not be taken when you already have a low blood pressure or while pregnant so this is why I suggest asking your physician for advice.
Basically he or she will either prescribe you some real diuretics (e.g. medical drugs) or approve using some natural diuretic foods. If he or she does so, here are some diuretic foods and recipes recommended by others.
But make sure you contact your doctor first and ask him for advice.