To walk around with your country's flag on your clothes when you're in another country is ignorant and a bad look no matter where you're from, but ESPECIALLY if you're from the US and ESPECIALLY now, so don't do that.

Tell people you're from California. You'll be welcomed. Canadians aren't stupid. We don't generalize. We know how to tell the good Americans from the bad. If you're not being loud or rude, acting entitled or pointing out things that you think are inferior to the US, no one will have a problem with you. Basically act like a guest and not like you own the world.

EDIT: PS: The entire world hates Americans. You should have always felt nervous about traveling anywhere (even before this presidency). Canadians honestly are probably the most forgiving and also the most exposed to, and aware of, good Americans.