Advice? would the doctor know if you masturbate?

Already does: Essentially all males during and after puberty enjoy their bodies while they're alone. The large majority of females do as well. This is not any kind of a health problem or anything you should be embarrassed about. Somebody who's ignorant may be making this a problem for you, but hopefully your physician is a realist. If not, he/she should not be practicing.

It does not matter: I doubt any real physician would care. I would bet that almost all physicians will assume you and everyone like you masturbates. Masturbation is a common, natural, and healthy part of the human experience. Almost all men and nearly as many females masturbate. Why would you be worried about this?