I am having a watery dark brown period with severe pelvic and back pain. what could this be and what should i do?! i have the nexplanon birth control.

You need to be seen: If your Nexplanon is administered properly and timely then we need not worry about ectopic or miscarriage but a pregnancy test should still be done as part of your evaluation. It is more likely that you have an infection. In any case, you need a pelvic exam and some laboratory testing ASAP. If you cannot get in to a primary care doctor or gynecologist in the next day or so then go to the ER now.