I am over week, have taken four pregnancy tests and all are showing up negative. Could I be pregnant?

I am now over a week late. I am never late; my periods are like clockwork. But, according to four pregnancy tests taken since I missed my period, I am not pregnant. I did take 2000mg of Zithromax for an infection a little over a week ago and am under a good bit of stress, but antiobiotics and stress have never affected my cycle before. I have two beautiful girls, both who were conceived the first month we tried. I also got a positive pregnancy test on both of them three to four days before I missed my period. My husband and I practice natural family planning. The hormones in birth control mess me up and the latex in condoms give me awful UTIs. Has anyone else been pregnant and received multiple negative pregnancy tests?