What Causes Light Periods?

I ve been on mercilon for 3 years and last cycle i missed a pill and i took it as soon as I remember. I am suppose to get my period last Friday, I did get a very light period along with all other symptoms that i normally get from my period. But the period was too light that I am kinda worried. I checked implantation bleeding should be brownish in color but mine is bright red. Would I be pregnant?