Please help!

My period is due around the 14th-16th of every month,and have been since I've had my first child in March. I'm now 3 weeks late, I've taken 3 pregnancy test and there negative, I didn't find out I was pregnant the first time till 6 weeks....is it to early to show up on a test and tell me if I am? or should I be worried? What should I do.....