If your carseat is uptodate, there is no reason why it should be the carseat. I have never had this happen, but I guess it could. Was there something in the carseat under her or was she strapped in too tightly? I would think the baby would let you know of her discomfort before such a bruise could occur as well.

It is possible that she could have gotten the bruise before you put her in the carseat and didn't notice it.

I would really be concerned if the bruise does not fade or if she seems to not want to move her legs. If nothing else, you can take a visit to the pediatrician just for peace of mind, otherwise, ask about it at your next checkup.