I need advice!

This is my first time on here and actually asking others for advice so here it goes. I have already had 2 healthy baby girls and now I am 5 days late for my period. I've always had normal periods (28 days) but I've took a million pregnancy tests and they are all negative. Even took a urine test at the doctor and still negative. I've seen lots of post about low hcg levels but I just don't picture that actually happening to me. Just thought it didn't happen very often, but anyways. It's a scary thing being late and not knowing why. My doctor told me to come back in a month (which I thought was stupid to wait that long) but maybe someone else has been through this and could give their opinions! I would love to have another baby, just hope it's nothing bad or anything...