Oh boy..I went through exactly the same experience only, in my case, it was not my DiL who was being snappy and vitriolic, it was my own son; After the first realisation that our relationship had somewhat shifted and that he was now being hurtful, I just chose to remove myself from the picture entirely and let the two of them be, so that things would not get to a point where it would be too difficult to mend them.
But it was basically what you describe: a complete U turn from what our relationship as mother and son used to be. So, I would say it is not hormones or post natal depression and obviously it does not just happen to young mothers; apparently fathers too. I think it is due to the anxiety and stress of a totally new situation of extreme responsibility , a new situation in which one or the other in the couple will maybe not cope so well. In your case, as in the majority of cases, it is the DiL. In my case, it was my own son.
I don't think there is anything to be gained by continuing to try to approach them or trying a different approach or whatever. Just back off and let them be. I trust they will come to their senses in time, and if they don't, then they don't. Hard, I know, but do you really have a choice?