Thanks for the additional information although I'm sorry that she's still not feeling well.
An anal gland issue such as an abscess or impaction was the first thing that came to mind when I read your question so great minds must think alike, I guess.
And, as you probably know, this particular problem is much more commonly seen in the smaller dogs although really any sized dog can develop them.
If she'll let you without getting bit, I'd apply a warm compress to the area. Alternatively, since she's so little, you could sit her in a sink with warm water and Epsom Salts which may soothe the area. Then dry her off and apply over the counter antibiotic ointment to the area, if you can.
It might help to wrap her head/body in a towel so that just her rear is available for you to evaluate and treat (again, if she'll let you).
Since she's not currently taking any medication and if she's not vomiting, then she could be given 81 mg (low dose) aspirin twice a day with food to avoid stomach
upset. This drug not only has anti-inflammatory but anti- pain properties as well which may provide relief.
Other possibilities include a hot spot which means she excessively licked her skin until it's raw
or possibly trauma to the tail. All of the suggestions I made above would still be of help if something else is going on.
I wouldn't necessarily consider this to be an ER type of situation although I'm certain that she's uncomfortable. Waiting until tomorrow to have her seen should be fine.
I hope this helps and that she's feeling better soon. Deb