I am going to tread lightly here. I am not a doctor but I have three children and 2 of them have a lot of medical problems. I do not know of any medical problem that would only cause bruising around the anus, that does not mean that there is not one. I would assume that your daughter goes to daycare since you said that both you and your husband work full time. I am frightened by what you are describing. I would be looking into the possibility that maybe someone has sexually abused your precious child. Do you notice her getting upset when you clean her bottom? Anything out of the ordinary? Do some questioning and I highly recommend that you take your daughter to the doctor to find out what is wrong. I will keep you and your daughter in my prayers. I even had trouble typing this to you, but it had to be said, it is a possibility that you have to check out to protect your young child. I hope that there is another explanation but your description of her anus sounds like the possibility of molestation needs to be explored.
God Bless You and please do not take offense to my letter, just check everything out and protect your child.