Thanks for the query.
The possible causes for your problem are:
imbalance leading to spotting and other symptoms
as urine pregnancy test kit may not detect early pregnancy sometimes.
-Pelvic inflammatory disease
So consult gynecologist once, get examined and if needed go for ultrasound
By this workup the possible cause for your problem can be identified and treated.
For more details you can ask me through: http://www.healthcaremagic.com/doctors/dr-sree-gouri-sr/63429