I am very sorry for your situation.
I will also tell you that to me, after being through the same thing, and seeing it here time and time again. I think that there is a VERY GOOD chance that there is another man. This is just the way that things happen like this. The immediate distancing is because they feel the pull, but cannot take the guilt.
I don't know and I definitely feel for you. Read here and keep yourself as strong as possible. Believe it or not, you will continue to progress regardless of what happens to your marriage. Read here and post your feelings, questions, and concerns about anything and everything, NO MATTER HOW SMALL. If nothing else, you will find that you are not the first, second, or even 500th person that this has occurred to. And with the people that have gone before you, you will find strength, empathy, sympathy, and respect.
There are no words to help you to understand that WE UNDERSTAND. It is just a horrible place to be, and the fact that you see it so much differently than she does will make it that much harder to understand. The only advice that I can give you right now is don't try to understand too much.
That being said, I know that you will try to figure it out. You will spend every waking moment trying to come to an understanding how this could happen. How could she do this? How could she do this to your family? How could she do this to your children? What is she thinking? IS she thinking? What can I do? What could I have done? Did she let me know? Was it me? Is it her? I can change. I will change. Nothing is worth this.
You will go over these things time and time again over the coming weeks and months as you attempt to put some sort of fence around this horrible occurance in your life. You will feel completely powerless and out of control. And this, my friend, is exactly what you are. At least concerning her. You cannot control her actions, thoughts, or desires. You will try to do things to "win" her back, but you must try to keep yourself sane. For yourself and you daughters.
That is the simple fact that will have to become the predominate thought in your mind. Are my children safe? Are they as absolutely protected as they can be? How can I improve their situation and protect them from this mess? Those questions MUST be the preemminant thoughts in your mind. They will give you something that you can control. They will give you something to focus on.
Look at your wife's behavior closely. Snoop a bit, and I am definitely not a snooper. But I am just afraid that, although I hope that I am wrong, that the signs you are talking about sound like there is another man involved. This information, if true, might help you to understand things a little more cleary. Read His Needs/Her Needs, it is agreat help regardelss of whether there is an affair or not. It will give you things that you can actually DO to improve and perhaps save your marriage. See if she will go to marriage counseling. If not, YOU STILL GO TO COUNSELING !!! for yourself and your children. You and they will need it.
Take care and post often. We are here and are at various stages of going through the same types of things.