Unless you are a small baby - you will not have a gap in your head that would indent if you are dehydrated or through cabin pressure- you will have solid bone - if it had suddenly became dented, I think that you would be experiencing some kind of pain. I would think that it has always been there. I am not aware if there are any illnesses that would cause this to happen.
Hmmm, i dont know where you went, but maybe a little insect from the other country made a nest in your head. It may have laid some eggs under your skin so get it checked out...