Pregnancy spotting no apparent reason

Amy
I have spotted my entire pregnancy. For a while I had a subchorionic hematoma which is now gone. Then I was diagnosed with placenta previa, which has since migrated. I have spotted brown for weeks. Now back to dark pink. It's never a lot of blood, but enough to make me worry. What are some other reasons this could happen?
I had an ultrasound yesterday (#6) and they had no answers for me other than I have two fibroids 1.5cm square. Can fibroids make you spot? I'm 21 weeks tomorrow.