You are very welcome, Laura.
If the lump is small but hard and sore, I would advise having a peek when you can to make sure this isn't just a tick having a nibble of Indy. If it is, you may just need to pop into the local pet store and pick up a tick remover (example
Otherwise, for firm lumps this small in a cat her age, we'd be considering pimples (from bacterial infection), skin colored lumps (in response to allergies or irritation), cysts, warts, skin tags, or potential abscesses (less likely this small but a potential in this area). Of course, with masses, we always have to consider sinister cancerous ones, but based on description alone this does sound unlikely here. Especially since our ticks, abscesses, bacterial and allergic lumps would be the ones to cause discomfort when handling.
So, do have a check when she will let you and if it is a tick, then removal would be ideal. Otherwise, you may just want to salt water bathe the area to reduce any risk of bacterial issue until you have her seen.
All the best,