Could i be pregnant?:-?

I havent got my period this month. My last period was on september 13 . I experience some watery dark red bleeding for 1 day ...1 week an ahalf after my period on sep.13. I thought it wad implatation bleeding . I already took two pregnancy tests but both came back negative:( i still havent got my period this month... Could i be pregnant? How long does it take for a pregnancy test to be positive ?