I Started Bleeding Sunday Dark Blood Then Changed To Bright

I started bleeding sunday dark blood then changed to bright red this went on some of sunday and then monday i was barely bleeding..i have spotted for 2 days now only when i wipe really..no clotting..no cramps..which is strange my period is usually very heavy and lasts 5-6 days with lots of cramps and clotting...wondering what could be going on..could this be implantation bleeding ? I did a pregnancy test monday however it came back negative. There is a definte chance of pregnancy as spouse and i had sex on fertile days.