Technical term for the loss of words in your mother-tongue when speaking a different language
It seems to me that it'd be a type of a tip-of-the-tongue phenomenon caused by bilingualism. If you don't use German regularly, it might be attributed to language attrition, but this seems unlikely if you're still being exposed to German more than English.

http://en.wikipedia.org/wiki/Tip-of-the-tongue_phenomenon#Effects_of_bilingualism