I would check that she isn't tooo warm during the day. Especially if she is out in the sunshine (thank goodness we've finally gotten some sunshine). Is this something recent, like the last day or two?? Or has it been going on for awhile?? We tend to just blanket at night, since it is warmer during the day....that is of course if she is out in the sun during the day. Also, if she has been out in the rain that we've had...make sure the blanket is waterproof, otherwise she will be soaked through.
But my bet is she is getting too warm during the day, since we've had nice weather up here (well at least for the last couple of days).
~kathryn