Swelling on dog's nose?

I have a 12 year old lab. Two weeks ago, she had a hard knot on her nose and overnight it became quite swollen. I took her to the vet in the morning and we assumed it was a bee sting. A steroid shot worked just fine and her nose was back to normal in 24 hours.

This morning, the knot is back in the exact same place. It is about the size of a silver dollar. The soft swelling of the whole side of her face hasn't come back yet.

Does anyone have any idea what this could be? It doesn't seem to be bothering her at all, but I am just very puzzled that it returned. Of course I will contact the vet, but as it is the weekend and not seeming to bother her, I will take her Monday unless things get worse in the mean time.