Hello and thank you for asking your question on just answer. My name isXXXXX will try to help.
There is a type of tumor, in young dogs, called a histiocytoma.
They are benign masses that usually resolve in 3-4 months.
They are usually red and round.
I could look at your picture but I will still recommend that you have a vet examine the mass. Especially if it has been there for a few months and it is growing.
The vet will take a sample of the mass, look at it under the microscope, and let you know if any cells look suspicious. If there are abnormal cells, the mass can be removed right away.
If it were a pimple, pustule, or localized spot of infection it would be resolved by now.
She could also have a foreign body, such as a wood splinter, stuck in the mass. If the cells look like inflammation under the microscope, the vet may opt to do some further diagnostics to make sure there is nothing in the mass.
To help calm the inflammation, you can apply hydrocortisone cream, or spray, every few hours. You can apply neosporin antibiotic ointment to help fight infection. Both are non-toxic if she licks them.
Whenever a skin mass is questionable, it is always best to have it checked while it is small. Some people wait until the mass is so large that we can not remove all of it even if it is cancerous.
I hope this helps.