My last period was in October 17th, so I went for a pregnancy test but it was positive . Few weeks later precisely on d 3rd of November. I started spotting and it turned to heavy bleeding so I went for an ultrasound and they couldn't detect any pregnancy, except that I have multiple fibroid. But the pregnancy test is still reading positive. What could be the cause

The fibroid probably won't cause an increase in the hormone that most home pregnancy tests test for. It could be that you had conceived and implanted but for whatever reason the embryo aborted it self and your hormones have just plateued and are going down.

Regardless you can't really do anything at this point but wait. If you miss your next period then you might want to go for another ultrasound and check for ectopic pregnancies.