is it stress?

Today was supposed to be the first day of my period which I haven't gotten yet. I've taken 3 pregnancy tests so far and they've all come back negative. I am stressed out but I'm wondering if that's what's holding it back me not getting my period or because it's still too early if I was technically pregnant right now I would only be just shy over 4 weeks I've had a problem in the past with my numbers not being high enough so I'm wondering could that be it?