Is it normal to fall for someone when in a relationship

I’m studying overseas with a small group of people and have been in a relationship for 5 years. There is a person here however who I think I have developed a crush on. Like, a big crush. I’m not planning on acting on it though of course but sometimes I imagine to myself what would happen if I did..?

Is this normal when you’re in a relationship? I feel guilty but I really, really can’t help it.