Round spot behind right leg getting bigger on Chihuahua now losing hair

We have a 1 year old Female Chihuahua, Penny, that has 1 spot that is growing larger and now losing hair. Doesn’t bother her, not flaking or red, it’s just there. We took her to the Vet and she assured us that it’s not a ringworm or anything to worry about, that she’s healthy. That was about 2 months ago, but it’s getting bigger. She’s full of energy, loves her Mommy, and is such a joy. Anyone experience this? I’ve researched it, but come up with multiple spots, bumps, flaking, red, irritating, but nothing with no systems, with just being there and growing. Please help.