Without trying to cause unnecessary panic, the answer to this question unfortunately has to be yes, it may well be serious. There could be a whole array of different causes for this lump, some of which may be harmless, while others could be potentially life threatening. It is best to consult with a vet though, as I am not a trained professional.
The cause for this lump could be anything from an allergy to insect bites or stings; from an injury to a hernia; from a benign tumor, like a Lipoma, which is a collection of fatty tissue, to cancer. More detailed information regarding possible causes can be found at
Pet Education.
In any event, the cause of the lump has to be properly diagnosed by a vet, in order to determine what type of treatment will be required. If the lump is growing, or if the cat is in pain, refuses to eat, loses weight or shows other signs of distress, it is imperative that she is taken to a vet for a proper diagnosis.
To start with, the vet will examine the cat. He will probably ask a few questions regarding the cat's history, such as how old she is, whether she has been spayed, contact with other cats, etc. As lumps can be very difficult to diagnose by touch alone, the vet may then decide to use a Fine Needle Aspirate, or FNA, procedure to establish what the lump is.
This simple procedure is usually performed without any sedation or anesthetics. A very fine needle is inserted into the lump and fluid or cells from within the lump are drawn up into a syringe. They are then examined under a microscope and will indicate the cause.
As I am not a trained veterinary professional, my immediate advice is to visit a vet as soon as possible.
