Is it normal that my gf's mom's belly and butt turn me on?

So, I was helping my gf and her mom with some stuff, and I noticed her mom was wearing a really tight shirt and isn't as skinny as I thought she was. I noticed I had a hard on because I saw that she has a belly like a really sexy one, but still, is that weird? Like I get such a hard on with this woman. I ask this because I don't know many guys who get turned on by the stomach of a girl, or mom.