I once experienced this when I met a Dutch person during my travels in Canada.
When I told him I was from Belgium and he originally from the Netherland, I immediately switched from English to Dutch and he did not catch every word I was telling to him.
Since he left his home country a long time ago for Canada, he only interact in English and is mother tongue was sleeping somewhere in the back of his head.
So, it is possible to kind of lost your native tongue. When I was traveling for over a month without speaking Dutch that much and coming home after, I also noticed some little part of my native tongue was sleeping and I had to wake it up again :D