What Causes Blister Like Spot On The Arm?

This seems silly, but I woke up yesterday morning with a large bump on my arm. It looked similar to a blister in that it was filled with the same liquid but I got rid of it and cleaned it up. But today the same thing happened again, but on my other arm this time. I'm just curious as to why. My sleeping arrangements have not changed, but these blister like spots have appeared.. Do you know why?