What Causes Spotting After Periods?

I have a quick question about my menstrual cycle...
The last day of my period was on Sept 28th and it was a normal flow. Since then I have been spotting for over 2 weeks now. The spotting is brownish red in color and has a few clots in it. I dont think its a cyst according to some websites, but it looks like it could be implantation bleeding. I went to see a doctor and she said that it looks like early signs of pregnancy but that a pregnancy test would show that I was negative due to the hormones not being in my body anymore. Is this true and do I need to be concerned?