"Oh, you're thinking of pescatarian, I'm (lacto or ovo) vegetarian, which is vegan plus (milk or eggs)".
I've found that people think you're vegan (absolutely no animal products) or that you eat fish, but they have a hard time thinking of something in between this. So, I've found it easier to start my clarification with vegan and add on what you're allowed to eat. BTW, the reason fish isn't considered meat has something to do with the Bible, but I'm not Christian, so I can't quote passages for you.
But, if you're vegan, well, state you've vegan, so nothing that comes from an animal, ever. As for being malnourished, well, state that you've been able to get all your nutrients from plant sources, such as b12 from yeast, proteins from food combining, iron from green leafy veggies, etc., so you personally don't see the need for obtaining it from animal sources.