Your dog has a small lump on the left side of her rib cage is this serious?

Cysts, or masses, can be benign or malignant. See a vet will help. My dog had fatty masses that came from minor injuries early in life and they are not life threatening. They may become uncomfortable and the benign masses are usually easily removed.
Cancer can also cause the masses. That is way you should have a vet look at the masses.