I had this same problem last cycle. I got AF every 5th week like clockwork since Jan, then all of a sudden last month it never showed up. So I tested a few times, giving a few days inbetween and all were negative. I went to see my doctor who did an ultrasound. He told me I just did not ovulate that month due to my PCoS and gave me provera to bring on AF. I hope for you this means you are indeed pregnant. I looked it up at the time and found that it is possible for some to become pregnant and not get a positive test for a long while. It could be a faulty test batch.
So, your best bet is to get to the doctor ASAP and find out what exactly is going on. If you are pregnant perhaps the hormones are abit low. If that is the case the doctor can give you meds to help, but the sooner the better. Good luck and let us know.

-Windy