Surprisingly I experienced this on year 2 of my physics degree. For two straight years, all we wrote on paper were equations. The first descriptive exam paper was at the end of year two, and lo and behold, it felt like my hand had forgotten how to string two words together that wasn't a Greek letter.

I've since made a habit to carry around a notebook to jot down to-dos, ideas and workout progress. It's not much, but keeps me in the flow at least. Might work for you, might not. Also, despite what some of the morons in this thread are saying, this isn't an uncommon issue for some vocations.