Well dogs can go through mood changes and phases like we can. Also, some dogs are more laid back and easier to show their expressions than others.
Maybe the sudden change of environment caused your dog not to sleep like usual and was just tired. If it last for a long enough time maybe your dog is sick, but I couldnt say. You know your dog better then us!
Even though he stayed with Grandma, it wasn't the same. Routines weren't the same, he was staying in a place that wasn't his own and he probably missed you! My cats were the same way when I went out of town for the weekend. All should be back to normal in a few days.
There are too many things that could affect behaviour .. The things the come to mind is the stress of his being displaced from his home and family (even though he may have been treated well, he may feel out of sorts for having to share the limelight with another dog or may simply feel sad and lonely because he is displaced from his home) or he could not be feeling well .. If his diet had changed it could be a reason why.
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