Possible Implantation Bleeding?

My period is due April 2/3. So I went to the bathroom yesterday and when I wiped there was old blood (brown). I thought for sure I'm getting my period early. However, there was no blood throughout the day. My symptoms so far are Diahrrea, Sore Nipples, Slight Nausea, Cramps, Lots of CM and Headaches (which are also pms symptoms). My question is how soon after should I test after possible implantation. I've actually caved and tested yesterday and today morning I just cannot take looking at another negative.