Hi Kristen
A sore in that location may be just a sore . . . or it may be something more serious. I know that isn't very helpful but there is no way for me to tell if this is a simple or more complex problem without taking a look at your Jack Russell.

If you want piece of mind, then spend the money and get a vet to have a look. If he/she says it is nothing to worry about, then you can put your mind at ease. If it does turn out to be more serious, then at least you got it looked after.

I'm sorry I couldn't be of more help.

Good luck,
drphilvegasvet