I don’t think the way you masturbate is especially odd. Its the same as frottage or the Princeton rub, you’re just doing it without a friend. Depending on how vigorous you are, its as likely to cause damage as jacking off with your hand. Many men who jack off “normally” use a grip so tight they cause damage, at least you haven’t done that. Regardless, masturbation doesn’t damage your penis, and a damaged penis doesn’t shrink.
The best thing you can do is go to a doctor, check for physical damage, and raise your concerns. If there is damage and he finds it, there’s a chance he can treat you. If you put off a doctor visit out of embarrassment now, you will end up regretting it for a very long time if you can’t be treated later.