It doesn't have to be FMU if you've held your urine for at least 4 hours with little to drink. This will allow the hCG to build up, but if possible FMU is always recommended as it contains the highest amount of hCG.
Sometimes your HCG levels are not high enough to register in a urine test... I would request a blood test to be sure, you will have to wait a few days for the results but at least you will be sure about whats going on....