It can be. I was told once that sometimes the blood will 'pool' inside your uterus and form a clot instead of just flowing out with the rest of it and after a while they will come away. You have to remember too that your uterus is still shrinking and healing from where the placenta was as well, so as it gets smaller it is going to force out anything that may still be in there. If it is not accompanied by pain, fever or heavy bleeding, then there should be no cause for alarm, but keep an eye on it jic.