My English students here in France are sometimes surprised when I can't immediately find the English translation for a word that I understand perfectly in French. Sometimes it's because there isn't any real translation -- an excuse I may have hidden behind a few times, to tell the truth!
Other times, however, I just can't remember the English word. I knew it once, and would of course understand it immediately, but it isn't in my brain anymore. Apparently, this phenomonen is called first language attrition. According to researchers at the University of Oregon, our brains may close off access to our native language as we become fluent in a second language -- a type of compensation process.
I originally found this information from a press release on EurekAlert, a site devoted to scientific research news.
Now I know how to explain to my students why I'm not always a walking dictionary!