implantation bleeding or something else?

[jakejoshmacie]

Hi,
New here.
Me and my partner have been trying to conceive for eight months. No luck.
My last period was the 5/7/14 and next due in four days (Friday)
Six days ago i started spottin, it was purple/brown and quite thick but not a lot of it, not enough for a tampon.
for the last six days I've had a little spotting daily but very very minimal.
Done several tests thinking it may of been implantation, two had nasty evaps (FR and a Tesco own brand) and two different tests today but all very negative.
I never usually spot between periods and going out my mind as very tired and had awful backache the past three days, i don't know if im poorly with something else or possibly pregnant.
Any advice or opinions? Xx