Why have i gone from passing very small amount of urine to passing none in space of 4 days? very uncomfortable feeling in stomach and had fever.

This is serious: Not passing urine at all is an emergency. It means either that your bladder is not emptying or your kidneys not functioning. The fever could be due to a urinary tract infection. You need to seek medical attention asap.

Need exam: This sounds like urinary retention with a very distended bladder and a urinary tract infection. Your bladder may not have emptied because of outlet obstruction or a neurologic disorder. You should be checked by your physician asap.