I am sorry to hear that Reba has a marble sized swelling just outside of her anus.
With your description I believe that Reba is suffering from full or infected anal gland(s) or impaction of the gland(s). Rarely this can be the sign of a mass in the gland. Anal glands are scent glands located at the 5 & 7 o'clock position around the anus. If for whatever reason the glands don't empty properly or the discharge becomes too thick the gland overfills, may become infected and is painful.
If the glands are unable to empty eventually with nowhere else to go the gland causes external swelling and may abscess out through the skin. This is a common problem for dogs and seems to be more common in overweight dogs that are fed lots of table scraps or eat a diet low in fiber. These glands are normally expressed when passing stool but if she eats a diet low in fiber her stools may be too soft, small or infrequent to empty them.
Dogs will scoot their bottom on the ground, even sometimes spinning in an attempt to empty their glands, or they may lick their bottom excessively. If she cannot reach her bottom very well she may just look at it quite a bit and grumble.
As far as long term prevention if this becomes a recurrent problem for her I recommend weight loss if she is overweight.
I also recommend decreasing or stopping entirely table scraps.
A diet higher in fiber, such as a weight loss diet or mature diet, may help. Sometimes a prescription food very high in fiber like Hills r/d or Purina Veterinary Diets OM is needed.
If she will let you touch the area she would benefit from a cool compress to the perianal area, patting dry and then applying a light coat of cortisone cream (like cortaid) to the area a couple times today and until you can have her anal glands checked.
Unfortunately the more she licks and scoots the more painful the area will be and the more tissue trauma she will cause. If the area appears red and she is licking then an e-collar (lampshade) is a great idea until she can see a veterinarian.
I know that she is uncomfortable but if she is still eating and drinking normally today then this isn't an emergency, just see her veterinarian tomorrow to check her anal glands. Do not feed her breakfast the day you take her in as they may need to sedate her to treat her. Overfull anal glands can be very uncomfortable.
If she seems absolutely miserable today though then an emergency visit today is your best way of getting her immediate relief.
The other reasons for perianal swelling include perianal hernias, perianal adenomas (more common in un-neutered males but possible with a female too) or a mast cell tumor.
Best of luck with Reba, please let me know if you have any further questions.