Why did I have an erection at the store?

I was going to the thrift store to shop around looking for items where I can make a profit. I I was in there for maybe two hours or so and all of a sudden I had a huge erection. I had to exit out the building quickly, drive home and went to the bathroom, pulled out my pants and started masturbating. Is this a common thing for men? I'm in my late 20's by the way.