Submodule confusion when checking out branch

I added a submodule to my project like this (all from the root of the project)

git submodule add git@.....   path/to/submodule
git submodule init
git add path/to/submodule
git commit -m 'I added a submodule!'
git push

All looks good and 'git status' reports 'nothing to commit'

However, I now cannot change branches. On checkout, I get...

"error: The following untracked working tree files would be
overwritten by checkout:"
(followed by a big list of all the files in the submodule)

Where did I go wrong and what can I do to sort it?

Thanks again!