Bonny,
I am not sure that her flanks have sunken, althought it appears that way. What I am thinking is that in reality the baby has "dropped" down which is making your mare appear to be thinner in that area.
So what you are seeing is a change in where the baby is being carried and there are normally 3 of those changes that I notice which seem to occure in the month or so before birth. The baby will "drop" down first, then move back, and then finally will rotate into position in the hours before birth. Sometimes these changes are subtle, other times they are very obvious, and it always depends on the mare and the fetus.
Like I Shelly said, it is obvious that there is a change in her shape and that things have "dropped".
As far as being jumpy, since she is a maiden mare, she doesn't understand what is happening. I am betting that the baby is kicking and just generally causing havock due to it's cramped quarters and she doesn't understand why she is feeling the sensations that she has.
I wonder, does anyone know if mares can have false labors like humans can? I only wonder because I know that mares can actually stop the early stages of labor if necessary for the safety of the foal, so would they be more likely to have "false alarms" because of this?
Also, being cranky is normal as well. She is after all carrying all that weight and just looking at her makes me tired. Laughs. As far as things coming to a halt, they haven't. There is obvious changes in the past week, but more changes in the bag, which are the most obvious, may not be so obvious until closer to foaling since she is a maiden. So don't get discouraged as you are doing everything right by just watching her like you are doing.
Do keep an eye on the area immediately on either side of the tail head where the tail joins the body. If you press lightly next to the tail head, you should see how easy (or not) it is to move the tail head. The closer to foaling, the more like jello this area gets, and the easier the tail head will move. This will probably be one of your changes that you will start to notice before long. The muscles in this area must relax to allow the baby to be born. On maiden mares, this area may remain firmer longer, at least that is my experience.
Can't wait to see how she progresses.