Body language when a woman is standing talking with you she puts her hands and arms behind her back a lot?

It probably depends on the woman. I have a habit of doing that but that is because I am in the military and I am still used to standing "at ease" when talking to those in my chain of command. I especially do it if I am in a new/uncomfortable environment/situation and don't know how else to stand. It may also be because she is shy.