What Causes Missed Periods And Negative Pregnancy Tests?

Hi, My LMP was on 7th dec 2010. Me and my husband are actively trying to concieve since 6 months. In Dec we tried every other day. I have missed my period for 3 months now and had an ultrasound on 5th of feb.But they could not find anything in the uterus. Since then I have got both positive and negative home preg tests. I am confused. I dont know what to think of this. My doctor does not seem to care or do further tests as urine tests at doctors office was negative.Please guide. Thank you