Hello,
It is normal for periods to be absent for sometimes 4-6 months after delivery. This spotting can be due to your periods coming back. But if you have had any unprotected intercourse then check your pregnancy status. Sometimes ovulation returns before the return of periods.
Most home pregnancy tests that check urine beta-HCG are sensitive to 20-25 mIU/ml of HCG. The test can be positive anywhere from about 2-3 days prior to a missed menses to 4-5 days after, but usually it should be done 7-10 days past your missed period.
Otherwise you can get a serum HCG also done which is more specific than urine pregnancy test.
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 and please do keep me posted on how you are doing.
You may need to consult with your OB/GYN doctor. The dark brown blood is usually a sign of old blood.