Maybe you've seen this before. She's 18 months old. It is best to get all lumps in the skin or subcutaneous tissue of cats examined by your vet. Lumps can occur due to abscesses or trauma leading to hematoma or seroma formation, which are collections of blood or serum, respectively, under the skin. Even though they are usually due to some sort of trauma, hematomas and seromas are often non-painful.
Abscesses occasionally wall off and appear to be non-painful, as well.
Lumps in this region can be due to hernias. If the lump has always been present this is a strong possibility. Hernias also sometimes occur due to trauma and we have seen a couple of hernias that appeared later in life for unknown reasons. It is surprisingly easy to miss a hernia that has been present for a long time. I have noted hernias in the record and then had owners come in later with the cat, having just noticed the lump again and not remembering talking about it in a previous office visit.