Blood is red. Whenitissubjected to air, it becomes brown. Blood from your uterus does not clot like normal blood. So older bood coming from your uterus is brown.
Breakthrough bleeding and spotting the first few cycles of starting birth control pills are not unusual. This is often brown because it can take a while to come out.
Your back pain can also be a side effect of the pills.
If you think something is wrong, see your doctor.
Best of luck!