Yes, this is normal as far as I know, I have had 3 C-sections and after all three of them I passed rather large blood clots, some the size of tennis balls, my OB said this was completely normal because it is your body expelling the after birth that wasn't taken out during the birth, they do get some out but they can't get it all, while your in the hospital the nurses or doctor will come in periodically and press on your stomach, they are doing that to help your uterus contract and get rid of this. Also I was told that if you breast feed your child then somehow the hormones from doing this help your uterus contract better and get rid of this faster.
Having large blood clots after any surgery is not normal especially a c-section at least the patients I have seen. It could be tissue left behind that didn't get taken out per say. Or it could be the uterus is trying to expel tissue from the uterus. Call your OB Dr because he needs to know this, its very important. You don't want to get real weak from to much loss of blood. A lot of problems will occur from to much blood being expelled. Please rate my answer