That sounds like it could be an impacted and infected anal sac.
The anal sacs (or anal glands) are little glands located at the 4 o'clock and 8 o'clock position if you are standing behind your dog. They are normally about the size of a chickpea. They have a little tube leading from the anal sac to the anal opening, and what Mother Nature intended was that every time your dog defecates, a few drops of the fluid contained in these sacs gets deposited on the poop.
Sometimes, however, the little tube gets blocked and the anal sac cannot empty. Anal sac fluid is continuously produced by the body so the sac gets more and more full. Eventually it can rupture out through the skin.
Given how close the area is to fecal matter, anal sac ruptures seem to always get infected. Treatment requires oral antibiotics since the sac is under the skin and the only way to get the antibiotics to it is by the blood stream.
When I see patients with an anal sac abscess, I generally clean the area, start them on antibiotics and then see them back in a few days (once some of the swelling and pain have gone away) and try then to express the anal gland to get rid of the blockage in the tube. Often I have to see the patient a few times. Also, I like to see them a week or so after the antibiotics are finished to make sure the blockage (impaction) hasn't returned.
Here are some links to further information:
In terms of what you can do to help tonight, the most helpful thing would be for you to apply a warm compress to the area. If you have antibacterial soap in the house (Hibitane or chlorhexidene soap would be ideal) you can add a little to a cup of warm water. Put a washcloth in, then wring it out. Hold the warm, damp wash cloth to the swollen area for 10 minutes, rewarming it every 2 minutes or so. Wipe the area with a plain wet washcloth and pat dry.
If your Chihuahua seems painful and the warm compress is not giving her relief, you may be able to give ONE DOSE of aspirin (as long as she has no history of kidney problems and is not on other medications). Please use Buffered Aspirin if you have it, and give it with a piece of bread or something low-fat to eat, not on an empty stomach.
Here is a link that tells you about it, with precautions and dose:
I hope that helps you. If this has been helpful, please hit the green "Accept" button and leave feedback. I will still be here if you have further questions!