Didn’t want to say this without you answering my questions first, but just in case your not on again this evening…..
If it’s quite firm, non-painful, it looks to me like a fibrosarcoma. It’s a benign-ish tumour of the skin but very prone to being quite aggressive in its spread in the skin.
Best thing is to have it removed. The earlier it’s done, the easier it is to remove in its entirety. If left to get bigger, because you have to take wide margins of normal skin along with the lump it gets more difficult and in bad cases impossible. Belly is a good place to get this, lots of spare skin hanging around.
As this is me just looking at one picture, obviously also get this backed up by a vet who can properly examine it as well.
If its what I think, don’t worry too much, just get it removed ASAP. Simple op when the lump is small and if wide enough margins are taken by the surgical vet, removal can be curative in most cases. Occasionally because, as I said, it can be aggressive in spread, a few cells can be left behind so it may come back at a later date. But again, early removal is essential. This type of tumour in cats in 99.9% of cases is single location only, so just this primary lump and doesn’t tend to spread internally unless left to get mahoosive and extensive.
Hope that helps some!