I agree, I do not think masturbation is the cause of your lump. Some women can just develop cysts down there or have inflamed glands. You dont need to disclose masturbation to your mom if you are not comfortable doing so, but I would inform her of the cyst or lump. I'm not an advocate of lying but maybe tell her you noticed it a few months back while changing your tampon or while wiping after you go to the bathroom or showering. I would definitely at least get it checked out.
Hi and welcome. No, I don't think that is caused by masturbation or really, anything to worry about. It could be a cyst and we get those. Do you go for a regular check up with your doctor every year?