It's not clear what part the fingernail played unless it caused a cut, but even so, it doesn't sound like you got infected in any way from what you're saying. It doesn't hurt, not red, sore or hot to the touch. No fever? If it wasn't there before, it should go away.

There are such things as sebaceous cysts which are usually harmless and filled with a rubbery protein called Keratin. These sometimes go away on their own, or you can see a doctor and discuss removal.

There are glands on either side of the vagina which help lubricate and these could become blocked and form a cyst. They can become infected occasionally, but again, you say there is no pain.

You can also read this article by Merely Me, a writer and community leader for this site, about, Bumps on The Vagina: How to Know if it is Serious.

If none of this helps, you are still worried, or things worsen, you should go to a doctor and be examined for a diagnosis. Every parent knows that their children masturbate, because most of them did, so you are in a vast majority. Sometimes things go wrong with our bodies no matter what we do, or not do. So seeing a doctor for something like this should be no problem. I assure you the doctor is not concerned with telling your parents that you might be masturbating, only in taking care of your health. Good luck.