What Causes Missing Periods And Negative Pregnancy Tests?

I missed my period on the 21 of this month and keep getting negative results...how long should it be befor i take another test or am i just not pregnant? I have been pregnant 3 times. have a 36 month old and had to mc. my last mc was on jan 5 of this year. i am 21 this week and weigth rite at 200 pounds. i am 5 ft 3 in.