What could cause this?

Rachel

My boyfriend and I had sex 3 days before my period was scheduled to start. Now I am on vacation in a higher altitude place and my period is very light and a red and brown color (I usually have heavy flow). Does anyone know why this could be? Does altitude do this or something else? We also used a condom and I am on birth control. It has also been 3-4 days I’ve had this bleeding and counting.