Thank you for your query.
If the pregnancy
test is coming up negative, you should not worry as the test is 97 % sensitive.
Moreover, such post menstrual continuous spotting should be investigated.
You should consult a gynecologist for a check up and a pelvic ultrasound
Causes include hormonal imbalance, hormone containing medications, anxiety, stress, fluctuations in weight, polycystic ovaries
, thyroid issues, endometrial hyperplasia
The exact reason would be determined after a check up.
Meanwhile take care.