Just chill. Not all Jews are touchy or paranoid. Maybe you've just been unlucky in coming up against the most political ones. You could try to make a joke next time it happens, to lighten the mood. "You're being anti-Semitic!" "No, I actually love addition!" (SUM-itic.. get it?) haha :D
The ten tribes were not lost. The writings of the bible make it clear that the Jews of pre-Christian times knew their fellow Israelites were still beyond the river Euphrates. Read Asahel Grant's The Nestorians - or The Lost Tribes which goes through all the historical evidence available, from...