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Thank you for the additional information. It sounds like this may be a mammary tumor. Fortunately, it sounds like you've caught it before it's gotten large and it can probably be removed very successfully. I would make an appointment with your vet early in the week and let them check this lump.
The vet may try to aspirate it (draw fluid out). If they are able to do that, you know it's simply a cyst, and they may or may not decide to remove it. If there is no fluid inside, they will probably want to go ahead and remove it. The good news is that once these are removed, cats usually have an excellent recovery rate and few require any further treatment. If your girl is not spayed, you should do this when you have the tumor removed as the instance of tumors in unspayed females is much higher than in those that are not spayed.
Please let me know if I can help further.