7 days late????

I was suppose to have my period on the 16 and I still haven't gotten it... It has been at least 7 days and still nothing... I had a miscarriage in March and the dr said thing could be messed up for awhile I get that but my periods since then hasn't been messed up at all just off my one day or too normal stuff... I have taken 2 tests and each came back negative... How long should I wait till I take another one... Could I be pregant and my levels just be too low to test... I am just a wreck cause I really want kids and I hate getting my hopes up..