If you worked for the same entity -- that is your normal employer -- while on a business trip, lasting 2 months, your income is still US sourced, just temporarily working at a field office. Japan also considered it as an extension of your US job and therefore did not consider it local earnings and hence not taxed by Japan. You prepare your return just as normal -- not foreign earned income.
If on the other hand you were actually transferred to a foreign entity ( whether associated or wholely owned by your US employer ) then the situation and the answer may be different. So tell us a little more about who were working for in Japan ( just entity type information , not actual name )
