It could be anything really. I have had cats all through out my life and whenever they get a lump it always seems to be something else. However, I am aware of some common problems. Most of the time it is ether a bone-spur, benign fatty tumor (a lot of my cats have had those over the years), or, if you cat is a very hairy cat, a flee bite, or matted hair.
Seeing as it is under her, chin, I would suspect it is a fatty tumor. I would take Rukus to the doctor if you can afford it. Most (90%) of fatty tumors are harmless, but if it is growing, I would be worried, because the only common things I can think of are the flee bites, or the hair matting (which could mean many other problems.)
Summery: Take her to the vet if you can, anything that is below the skin of a cat the grows, usually isn’t a good, or safe thing.
Sorry if this worries you. But I know cats pretty well, as I have been living with them my entire life, as one point our house had 28, now we are down to four, maybe five if I get one of the Siamese kitten my friend breads (I love Siamese!) Also this is my disclamer: I am not a vet, nor doctor, or even close to ether of those, so don’t quote my words as being in stone.