Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that Abby is uncomfortable and has a bump near her anus that produces blood and pus when pushed on.
With your description I believe that Abby is suffering from full or infected anal gland(s) or impaction of the gland(s) that has now abscessed out through the skin near her anus. 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 abscesses 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 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 warm soapy rag compress to try to empty as much of the debris as possible and clean the area and then a cool compress to the perianal area to soothe irritation, patting dry and then applying a light coat of cortisone cream (like cortaid) around the area a couple times today and until you can have her anal glands checked.
You may need to clean the affected area gently several times until she can be examined.
Unfortunately the more she licks and scoots the more painful the area will be and the more tissue trauma she will cause. 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 regular veterinarian tomorrow to check her anal glands and prescribe medication to treat the abscess. Do not feed the night before after 7pm or breakfast the day you take her in as they may need to sedate her to treat her.
If he seems absolutely miserable today though then an emergency visit today is your best way of getting him immediate relief.
There are other reasons for perianal abscesses/infections, such as a perianal fistula, but the most common cause is anal gland troubles.
Best of luck with Abby, please let me know if you have any further questions.