4 year old daughter and abortion with second child

Patient: I am 23 years old and I have a 4 year old daughter. Last February, I had abortion with my second child. Apparently there was no complication. Now my last menstruation was on October 11 and until now I don’t have my period yet except for a spotting last November 2-5. I have tried taking the pregnancy test but the results are always negative. What could be the problem?