Surprisingly to many this is not odd behavior. My sister pretended to breastfeed her dolls, and my youngest son has done the same with his stuffed animals a few times.
This is just your child's idea of role playing. They make fake bottles for dolls and FurReal Stuffed animals. They make play food and dishes. These are all a form of role play.
I think that as long as your child doesn't take it too far, this role play is natural process. Like anything, if they get obsessive about nursing their dolls or stuffed animals talk to their doctor if it worries you.