If your Yorkie has swelling in this area, it is probably best that she sees a vet as it sounds like it could be either an abscess, a cyst, or even an insect bite.
Some of these can be treated easily, and others are a little more complicated...
Does it seem to be bothering her at all? Is she scooting?
The reason that seeing the vet is important is that he will have more of an idea when he sees it, but can also do a biopsy to see what is inside the lump. If it is pus filled, your little dog will probably need antibiotics.
I don't want to worry you, but there is a small possibility it can be a tumour, this happened to my family pet when I was younger. She was elderly at the time, and the lump did turn out to be cancerous, but given her age, they/we decided that it would be better to leave her alone and not have surgery. She went on for a good few years after that actually.
Obviously in a younger dog, even if it is something nasty, catching it early will mean surgery can be done quickly, and she will be back to her normal, bouncy self in no time.
Please try not to worry though, this is a worst case scenario, it could well be something that just requires a few pills or some cream. It may even be best left alone, but I do think it needs checking out properly.
I hope she feels better soon, let me know how you get on.