Whenever anyone begins something new, there seem to be an abundance of people that feel they have the right to criticize. We discuss this in several places because it is something many new vegans are not prepared for.
An important thing to remember in any situation is that you are under no obligation to explain yourself, unless the person asking is an officer of the law (or holding an AK47 assault rifle!) When someone asks you why you are vegan, or why you eat this or not that, you do not even need to acknowledge that they asked the question. If you do acknowledge the question, you are still under no obligation to answer.
Many people, including newly converted vegans, often feel they have some divine right to tell you what you are doing is wrong, and what they would have done in the same situation. Realise this is going to happen so you are not surprised when it does. Think through in advance how you want to deal with this sort of rudeness, and above all do not take any of it personally. People that do this sort of thing are trying to come to terms with their own insecurities. It is nothing to do with you personally.