What Causes Massive Dents On The Head?

I have massive dents on my head. I must have had them all my life but this morning i shaved off all my hair to get a better look and it looks formidable. I don't have the greatest memory and my hair never grows more than 4 inches. So, what is the cause of these massive dents/craters.



Age: 29

Weight: 14 stones

Medical history: good

Sex: Female