I find when they can lay out flat like that and rest for a while, that they are not in labor. I have never had one lay out flat, then have a foal later that evening. My guess is that it will not be tonight. She does not seem to be restless enough either.
Get some sleep while you can.
I have had them go down on their bellies and rest for a bit, but keep getting up then down and only rest on their bellies, then lay over sideways and procceed to push out a baby.
I have also noticed that for a few days before they go into labor, they will start laying out flat to rest. Then when the big day comes, they do not lay out flat at all.