What Is The Irregular Brown Spotting With Negative HPT?

On the day I was supposed to start my period, I had light brown discharge so I thought it would get heavier. It never did. The spotting lasted a day or two. I have cramps and headaches. I have taken 2 hpt and they were both negative. My period has always been regular and never late. I m also on my birth control. The spotting started Saturday, it is now Wednesday and I haven t seen any red. Could this be implantation bleeding?