You sure the indent is on your skull? Maybe it's just on your skin/tissue on your head. Stop using your headset for a while and see if it return to normal.

If it did not go back to normal, maybe it's just the shape of your head. I doubt headphone could squeeze solid bones like that.. Unless you have like.. Baby bones.. But if you're concern.. Maybe go to a doctor and ask him if the indentation is recent or.