ELI5: Why do other puppies try to feed off of my dog?

I've noticed that puppies try to feed off of my pug. We rescued her from a puppy mill were she had five litters of puppies, and the owners were going to kill her because her last litter was stillborn. My neighbor just got a puppy and it tried to feed off of her. I was wondering if dogs that have had puppies release certain pheromones that attract other puppies?