I've had the same problem online for years, when I translate something people presume I am using Google Translate (even though in one case said language isn't on Google Translate — I checked!)... Or got the answer off Wikipedia.

One of the funniest things was being accused of plagiarising Wikipedia, when I'd actually written most of the Wikipedia article on said subject. The irony... Wikipedia doesn't just use unpaid labour, it ends up undermining the people who wrote it.