What Causes Brown Spotting While On Birth Control Pills?

Hello, I have been on the pill for about 3 years now and skip my period most months. I try and have my period every 3 months which is what was recommended or when I see signs of spotting before I take the sugar pills. This month I forgot to have my period and now I am having brownish spotting for almost a week now. My last period was in november so is this just old blood for when I was meant to have my period? Just a little worried. Thank you.