You say that your period is regular but how long is your cycle usually? Meaning when should AF have showed? I'm also wondering if the shot you were given (was it progesterone?) might start AF but your body not display the usual signs since it's on "override" with the shot, KWIM? I don't personally know of any viable or non-viable pregnancies (e.g. chemical or ectopic) that wouldn't show at least some amount of hcg within a short time after implantation and with b/w they should be able to pick up even a small amount of hcg. Are you taking any meds or supplements that could lengthen or suppress your cycle? Sorry, lots of questions but no answers ... I'm just kind of thinking out loud here.