It could be a small abscess. Those are pretty common in cats that go outside. There is always bacteria on the skin. Any kind of wound allows bacteria to enter and cause an infection.
Since you can feel a scabbed area, there was probably a break in the skin.
Another likely cause would be a cat bite from another cat. Cat's have high bacterial counts in their mouths. So, a bite from another cat would likely cause an infection.
Since an abscess is caused by a bacterial infection, it is usually necessary to treat it with oral antibiotics. You would have to get the antibiotics from a vet as they do not sell them over the counter.
The vet can take a syringe and needle and draw some fluid out of the swelling. If it looks like "pus" then it is an abscess.
Abscesses are also usually painful when you press of them.
They can cause the cat to feel lethargic or even have a fever.
If the vet draws out a sample, and it is bloody, then it is a hematoma which means she has injured that area and caused a small area of bleeding in the skin. That should heal on it's own.
If she is acting completely normal, and the swelling is getting smaller, then you can probably just monitor her and the swelling should break open and go away in about a week. You can apply over the counter Neosporin as needed.
If she seems lethargic, or the swelling is not resolving or growing, then she needs to see a vet.
I hope this helps.
If you need more information, please reply again.