Hi,
Welcome to the ehealthforum. I am glad to help you out.
I would suggest you to do urine for pregnancy test or get your serum HCG levels estimated.
The HCG (it is a hormone) levels rise only in pregnancy. The serum HCG test is much more sensitive than the urine test.
So HCG presence will confirm the status of your pregnancy.
Once you are clear about the status of your pregnancy, you should consult a gynaecologist and get other causes for such dark brown periods ruled out.
The common causes can be overactive thyroid function (hyperthyroidism) or certain kidney diseases, intrauterine adhesions and polycystic ovarian disease.
Thyroid function tests, hormone studies and ultrasound uterus may be needed.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided.
For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care.