I have a long coated 11 yr old female Chihuahua. I just found a 35 mm round swelling on her left chest.?

your vet can . wellness check with your vet

Take it to a vet for a proper diagnosis/treatment.

Insight? You need to go to a VET not ask for guesses from strangers on the internet. Seriously?

I just got her shots recently. I thought it might have been the place they gave her shots.Thanks, Nae and CDog. That all makes sense.I am taking her to the vet next week.

It won't kill you to take her to the vet and find out what it is. It could be nothing.

Probably a harmless lipoma (fatty tumor), but keep an eye on it, and have your vet check it at your next visit. If it grows or changes, see your vet sooner.

Not every lump or bump on your dog will be a tumor. Some superficial bumps are just sebaceous cysts on dogs that are simply plugged oil glands in the skin and usually nothing to worry about. Skin cysts can be composed of dead cells or even sweat or clear fluid; these often rupture on their own, heal, and are never seen again. Others become chronically irritated or infected, and should be removed and then checked by a vet just to be sure of what they are.