How long has the lump been there? Is it getting larger at all?
Boxers are prone to cancer, so I would watch the lump too long. If it is fairly small, it could be a local reaction to an insect bite, but if it is large, I always worry about cancer.
Your best action is to have your regular vet do a fine-needle aspirate to get some cells to examine under the microscope. This is a simple procedure and involves just sticking a needle into the lump and getting some cells. These are put on a slide and then the vet will look at them while you wait. If there is any concerns, your vet will either send the slides to a pathologist for further interpretation or recommend either a biospy or surgical removal of the mass and then histopathology to find out what it is.
It is also possible that if it is next to a bony area (like the spine) and she had some recent trauma to that area, that the lump is bone inflammation. This would best be seen with an x-ray. As long as it didn't look like cancer, then anti-inflammatories might be prescribed.
If you think there was some trauma that started it, then you can probably just monitor it (take a picture or measure it so you can compare it day to day) for another week or two. Hopefully after a couple of weeks it will get a little smaller. If it is getting larger, however, I would take your dog in sooner rather than later.
That certainly is a possibility. I know my husband developed hard lumps on his back quite suddenly and then figured out they were caused by repeated roller coaster riding that day! With localized cancer, we don't generally see any systemic effects like weight loss until the cancer has spread. It isn't always painful, either, until it becomes quite large. But with the door flap history and sudden appearance, I think you can just watch the area for a little while.