Have you been checking every day inbetween? Or just the days you mentioned? Fertility monitoring is a tough thing to do accurately. I say that because monitors don't tell you that you are ovulating they tell you that you will ovulate SOON. Usually it's anywhere from a few hours to three days from a positive test. If you have been getting a positive the ENTIRE time from day three of your period to day 15 I would think that you were already pregnant.
The hormone that brings on ovulation is VERY close to HCG. As one site explains it: the two hormones are twins, with HCG wearing a hat. This means that an ovulation test with find both hormones but a pregnancy test only looks for the hat. You might just want to try a pregnancy test!
at 2:52 AM on Jun. 27, 2008