Hi nathan!
If possible, in cases like this it is very helpful if you can post photographs of the problem, so people can see exactly what you are describing.
It is probably an abscess, which is a pocket filled with pus. Check the area carefully for signs of injury- you may see a small scab or closed wound.
If she has had it for a week already, it is probably about to burst on its' own, but you can speed things up by applying moist heat. Fill a container with water as hot as you can stand, dip a cloth into it and wring it out, and apply that to the swelling. Re-dip the cloth when it cools off. If there are any scabs, rub them off when they have softened and that will allow the pus to escape if the wound has not closed.
After it bursts and you have gently expressed all the fluid you can, use a syringe and rinse it out with a solution of warm water and betadine until the water runs clear (no pus- the water will be brown because of the iodine). It may swell up again in a few hours or a day or so later, in which case rinse it out well again.
This time of year flies are not a problem, but in the warmer months it is important to apply fly repellant on and around the wound to prevent the flies from laying eggs in it.
Don't be concerned if she loses all the fur from the area- that is normal, and it will grow back.
I hope this helps. Good luck with her!