It wasn't your fault. Nothing to feel bad about.
It can be a tricky situation when you visit someone and they cook a meal for you. You don't want to be too rude by asking what's in the dish, but you don't want to consume something you aren't comfortable with either, and you want to be consistent in your beliefs and practices as a vegan. I've run into this issue countless times at my Mom's. Though she has been on again off again vegan and she KNOWS how important being vegan is to me (and the fact that dairy made me very sick when I was an omni), she is not a label reader and is very lax about what's in stuff, and I am very strict. For example she will buy gluten free bread that also has egg in it. She didn't think it did until I pointed it out because I know that Rudi's gluten free breads have egg in them. She once offered me a salad dressing and I read the label because I was suspicious of that particular dressing and sure enough it had stuff I wouldn't eat. I was probably a little rude reading the label in front of her, and she yelled at me for being so extreme. And well you know it didn't turn out so well as I yelled back at her for not respecting my needs and wishes. I guess I am a little pushier than some and less trustful. I am usually very upfront and investigative at restaurants too before I consume a product, but oddly I get more respect there. and it's strange because usually I am very passive.
I think it is a bit rude on the part of those making a dish for you when they know you are vegan (they did know right?) and yet still put animal products in it. But sometimes people do forget. It doesn't make you less vegan because it wasn't something you had control over. Hopefully next time they will remember now.