What is prolactin? does it actually stop you from losing weight while breastfeeding?

A hormone: This is a hormone that signals the milk producing glands to make milk. It has that single purpose. It does not prevent you from losing weight on an appropriate diet.

Hormone: Prolactin is a hormone excreted by your brain which stimulates the mild supply. It does not directly cause weight loss. It might cause weight loss in the sense that it stimulates lactation which generally uses up additional energy supplies.