Happened to me!!!!! First pregnancy, went in for a yeast infection, found out I was 5 months preggos, but I had a period also for 5 month, full blown AF too. Second pregnancy, I tested every 3 days for 9 weeks and had blood work done, everything came back negative until I had 9w3d and then bam, got the first + on a blood test, still - on hpts though. It does happen, very rare from what I have read, I just wish there were a concrete way of knowing either way instead of the sit and wait game and feeling preggos and always getting the -s until you are in the 2nd trimester already, ARG Good luck to you though :)
If you google low HCG levels there is a long thread that is two years long on pregnancy-info.net. On that thread are women who say they didn't get a positive until they were 9-12 weeks with little babies kicking away when they finally got an u/s. So it does happen, why we don't know but there are stories out there...just google and see. Good luck!