My bfs family is going to judge me 😭

My boyfriends family invited me over for dinner and I'm soooo nervous!! I've been vegan for a little over a month now (specifically plant based vegan, which is more restrictive, I try my best to avoid any processed foods) and basically the whole time I've been a hermit cooking for myself in my house. So My boyfriends family invited me for dinner this weekend and I know I will hardly be able to eat anything! 
And just in general going out to eat terrifies me because the last thing I want is for people to think of me as a snotty stuck up vegan. I do this for health reasons because for the longest time I was super sick after I ate anything, even after I drank water. 
How do I be polite while also sticking to my meal plan and diet?? I'm so scared that his mom will prepare a huge meal and will get offended if I can't eat any of it.