I know it's not the same, but I've always felt something like this about math. I've been a math nerd since I was 4 or less, so there are a lot of concepts that come pretty natural to me, despite being difficult for a lot of people; I think that's caused because I started so early that the knowledge is ingrained into my brain almost as deeply as my mother language (Spanish, by the way).

However, math is a lot unlike language, because, with math, knowledge grows infinitely; but a language is finite, especially with regard to the grammar. That's why I avoid using the term "bilingual" when I speak to another people, even if inside my head it makes a lot of sense.