Dead guppy - strange pump

[Blueberries]

One of my four guppies died. She died between Thursday and this morning. She seemed to have a bump on her right side. My other female guppy has the same bump. I changed out some of the water and added fresh water with some aquarium salt.
What do you think the bump is?

Since adding my four guppies my aquarium seems pretty peaceful. The betta and frogs leave each other and guppies alone.

Photos attached