Skipped period- breakthrough?

I skipped my period last month because I was on spring break with my boyfriend. I continued with my active pills and skipped my placebo week with no problems, until last week. One week before my placebo pills started (3rd active week) I started getting brown discharges ( I assume old blood) and eventually started bleeding. I am now on my placebo week and experiencing very light spotting of dark brown blood. Is this normal? Does anyone know why this happens?