What Causes Missed Periods With Negative UPT?

I had a miscarriage on 7/19/11; I bleed for two weeks and after the bleeding stopped, I got my regular period 4 weeks later. my husband and I are actively trying to get pregnant again. My period this time didnt show up at 28 days, I am now 10 days passed that. Could periods be messed up on the second one after a miscarriage? I've also taken 6 pregnancy tests and all have said "not pregnant". Not sure what I should do.