12 days late for period with regular cycles & negative pregnancy & blood test....what could be going on?

12 days late for period...been having all the symptoms and gained a few pounds about 5 days ago got a blood test done it was negative... as well as getting all negatives on pregnancy tests, have regular cycles too never this late... have been checked for pcos last year because of bad cramps and it was negative ...what would you do if it were you ?