soft lump on puppies head?

hey everyone!

so my husband and i got a new puppy, we’ve had her for about two weeks. in the last two days we noticed a large lump pop up on the top of her head. it almost feels like a bubble? it’s soft. she plays pretty rough with my other dog so i don’t know if it could be an injury from that. does anyone know what this could be? she has an appointment with the vet this week.