Please try git diff --staged command.
Alternative options available are listed below.
git diff
shows changes between index/staging and working files. Since, in your case, git add moved your files-with-changes to staging area, there were no modifications shown/seen.
git diff --staged
shows changes between HEAD and index/staging. git diff --cached also does the same thing. staged and cached can be used interchangeably.
git diff HEAD
shows changes between HEAD and working files
git diff $commit $commit
shows changes between 2 commits
git diff origin
shows diff between HEAD & remote/origin