This is probably a cyst or boil. Best to keep your hands off of it because you yourself could infect it. Agree to call your doctor to have a look and drain if you need to and you don't need to put a tampon in, this won't give your vagina an infection. And cortisone is not needed here either as that is an anti inflammatory and I think it's being confused here as something you put on for infection. It's not. This could heal all on its own but it is best to get it checked out!
Do not worry, I have had this. Call your OBGYN and see what they suggest. It will either drain or absorb back into your body.
Put in a tampon, so it won't cause an infection. If you don't have a tampon, tissue works. If you have it, put cortizone around the area just in case.