6 days late on other apps but on here my period doesn't start till the 21st....

Kelly • I was a teen mom before it was cool! Pregnant with #3☺
I know there isn't a set way to tell when your period is going to start but I don't know if I'm late or not. I've taken some tests all resulting in negative, but all I want to do is EAT! sweet stuff too which I normally hate. I've felt nauseous lately. Any advice?