What Causes Stomach Pain And Back Pain Before Periods?

My period has started on time. I am usually pre-mentstral and so it was no surprise that for a week before my due to start date that I was experiencing back pain and stomach aches. But now that I have started, the bleeding is very light and I usually have a very heavy flow. Also, the color is brown and red. What is going on? I am 19 years old.