Wow, what a tough situation! The thing that jumped out at me as you described your friend's behavior was Postpartum Depression. Especially since you mentioned that she has had anxiety issues in the past. PPD is more complex than people think and presents in many different ways (it's not just being "sad"). As someone who suffered from this with my first child, I can say that things that might seem "simple" to others was often very stressful and anxiety producing for me at the time. She could be manifesting these feelings in the way you described.
A suggestion for a positive way to approach the situation with your friend: find a time to get together with her alone and mention the weekend with some nonjudgemental comments, "you didn't seem yourself this weekend" or "you seemed really stressed...how are you doing?
That shows that your are concerned about her without critisizing her behavior and will give her an opportunity to talk about it. If she does have PPD she will be very emotional and oversensitive. She also may be very critical of herself both as a mother and a wife and feel like a "failure". Keep this in mind as you approach this. (going with the PPD assumption again) there are many resources out there-support groups, visiting moms that will come to your house, and therapists that specialize in this area. One big issue is actually getting out to a therapist and managing child care-dealing with these (even actually finding the therapist) can seem "impossible" tasks for some PPD moms and any help to faclitate evaluation/treatment from friends and family might be neccessary.
Sorry to run on but this is one of my "soap box" subjects. I was a pedi ICU nurse for several years, also have been around/cared for babies young kids for years and thought that this would adequately prepare me for motherhood. It was such a shock when I experiensed PPD and it took me a long time to come to terms with it-delaying getting help. Looking back from "the other side" of this experience gives some perspective:)
All that being said, there could be some other reason for her behavior and perhaps there pieces of this puzzle that you are not aware of that would explain the situation. Good luck!