Why am i gaining weight when i dont eat much? i asked my doctor to check my thyroid, dont know yet, is it water weight? doctor wont give me diuretics

Fluid retention can: cause weight weight but it's only one cause. Your doctor is doing the right thing by not giving you a diuretic w/o further information. Other causes include change in your metabolism, thyroid problems, a relative mismatch between the calories you consume and burn up every day. Sometimes it's a matter of underestimating how much food you're eating. Keep a food diary & f/u with your doctor.