I think the problem is that her anxiety is not soothed until she gets the message from you that says, "Oh hey, sorry I was asleep." During those 3 hours she didn't hear back, her imagination could have been running wild with all sorts of explanations, none of which were as predictable as being asleep. Obviously, this isn't your fault, but this might help explain her motivation.
She was probably surprised that you were asleep in the afternoon, and just didn't think about reasonable explanations like being sick. If she hasn't replied yet, she might be embarrassed that she made a big deal out of it at all and is trying to play it cool.