Is it normal that i sniff my thumbs?

Through out my day, while I'm doing something or when I'm just sitting around, I sniff my thumbs. Unlike most people that do this, I do not scratch or touch inappropriate areas. I just rub my upper lip with my thumb and smell it. It's as if I'm breathing in air, but from my thumb. It doesn't make any sense to me but I've been doing this since I was a little girl. Is this weird?