How common is spotting at 6 weeks?

LMP was December 1st. I'm unsure on when I ovulated, but I've been testing positive for the last 3 weeks. I've been spotting a few times for the last couple of days but don't have any pain. I had an ultrasound yesterday just in case and the tech said they could see the yolk sac but that's it so they thought it could be implantation bleeding. However, that seems unlikely.