four months no period and negative pregnancy tests

my last period was sep 12 2011. i spotted for a day in october 9 and ever since i havent started a period no bleeding or anything.. i did expience some cramping here n there for a couple of weeks but still no period. they told me i had hypothyroidism in november i started taking synthroid in december.. ive taken several tests n they r negative... i went to to my obgyn n he did a test there too n it was neg also.. he told me i should start my period within a month i start synthroid its four more days  to a month i started them n still no period. i dont have morning sikness or nothing like that i just get headaches here n there feel dizzy sumtimes n also lately i have felt movements in my tummy weird rite... well anyways the question can you be four to five months prego and not show up on a prego test. im goin to get an ultrasound this monday but i need expiences thanks... anything will help