This is very common. The main thing is not to worry. Most babies lose some of their hair, only to grow more later.
And some babies lose all their hair.
Your girlfriend could be correct - often it's rubbing on common parts of the head, especially if you always hold them against you the same way, but hormone changes also have a part to play.
I'm going to add to @Rory's answer because there's an important issue which wasn't completely addressed.
My gf's take on it is that since we (I) carry her too much, he head rubs in this area and the hair falls. When I hold her, there is definitely contact between my arm and this area.
Babies have big heads compared to their bodies, and the head weighs a ...
My baby was born with a full head of straight dark hair. At around 4-5 months it all fell out, he went bald for a few months, and then it grew back a pale golden blonde and slightly wavy!
His doctor said there is an aspect of it rubbing off on the crib, the carseat, due to cradle cap, etc, but it's more of a developmental thing. Some children noticeably ...