Oh, about those digi tests with the time indicator things...
I should have mentioned this earlier, so I apologize. The conception indicator feature of these tests tend to be quite inaccurate, from what I've heard. These tests give you your result based on the level of hcg in your system at the moment, which really can vary, but generally SHOULD at least double every 48 hours in early pregnancy. I have heard of women whose hcg was over 2,000, and they were only getting a Pregnant 1-2 on those tests...so they're not terribly reliable for some people. So those of you that have been getting a "not pregnant" after getting a positive reading, may want to try a different pregnancy test altogether, such as an FRER, or a CBE digi WITHOUT the week indicator.
My suggestion would still be to call your doc, and have him/her order you a series of beta tests, to see if your hcg is rising sufficiently, or falling.