I had the same problem with Makenzie, had heard that a sunken fontanelle meant dehydration, and her's was quite often sunken when upright, but only slightly - you could see it was slightly sunken but it wasn't like a huge dip in her head. If I laid her down again it went back to normal. That never happened with the boys, so it worried me at first, but she was getting plenty of fluids and heaps of really wet nappies and was happy and alert so I didn't worry about it.