Brown discharge week before period on birth control & other symptoms: could I be pregnant?

I am 21 years old and currently on the birth control Yaz and have been for almost a year now. Never had had any problems or spotting or anything with it I have not missed a pill but I do not take it at exactly the same time every day. I haven't had in about 5 weeks, but for the past week I have had moderate spotting changing from pinkish red to brown back to red. My period last month was perfectly normal. I also have had incredibly sore for the past week, slight cramping,severe lower back pain, fatigue, and some slight weight gain. My period isn't due for another 5 days but I took an early detection pregnancy test and it came back negative. Could I possibly be pregnant and the birth control is making it hard to realize this? I know some birth control has symptoms similar to pregnancy, but I figured some other insight wouldn't hurt, thanks!