How many days after your due date is good to do a Preg test?

Stefania
My husband and I been TTC for over a year and haven't gotten pregnant yet, it is sad when AF arrives and I get disappointed sometimes, we want this baby so bad and now I'm late 5 days, the weekend that just passed I got nauseous when smelled hamburger meat, didn't puke cause my husband made have ginger tea but I had diarrhea instead with stomach each and couldn't sleep too good that night. 
Could I be pregnant? Are those signs? When it's the best time to get a PT?