The good news is, cancer does not pop up to golf ball size in a day so I would not worry about that.
If it is NOT hot, red, or painful then it does not sound like an infection or an abscess.
It could be a cyst which is a fluid fill sack in the middle layers of the skin. It is usually caused by some kind of trauma to that area. A "hematoma" is similar to a cyst but it is filled with blood (like a blood blister) instead of serous fluid like a regular cyst.
A common cause of a soft lump, in dogs, is a benign fat deposit. They do seem to pop up over night. They are called "lipomas."
Since it does not sound inflamed, and it is not bothering her, there is really nothing you can do to reduce it or help her. You should not worry either.
Tomorrow the vet will aspirate a small amount of material, from the lump, and look at it under a microscope. Then treatment, if needed, will be planned. If it is a lipoma, cyst, or hematoma you will most likely just be asked to monitor it for now.