Is it implantation bleeding? Am I pregnant?

I have been tracking my BBT for the past month and have had your textbook temperatures- Lower before ovulating, then spiked once ovulated and after. Then 4 days ago I had what resembled light bleeding (5 days earlier then when I was supposed to get my period.. I am usually on time, or at max 1 to 2 days early... The bleeding was not like a regular period for me... My period is due tomorrow, I took a HPT yesterday and turned up negative... It if was implantation, shouldn't it have turned up positive? I am having a lot of nausea, which is what triggered me to think I may be pregnant... About my temp.. it went down the day the bleeding started, but has risen ever since, and now is above the coverline...any thoughts or ideas would be helpful! This is driving me crazy! Thanks!