What Causes Delayed Period While HPT Is Negative?

I am a 22yr old mother of 2. usually i have normal periods which are on the same day of every month. lately my husband and i have been having unprotected sex and so far i have missed 2 periods. i have taken numerous pregnancy test and they all are negative. i even went as far as going to the hospital and ask for a blood test or ultrasound but the doctors there think it would be unecessary( its not a very good hospital). I heard of people actually being pregnant without knowing but i dont konw if its true. can it be