Spacing after derivative operator in the commath package

The definition of \Dif in commath.sty is

\DeclareMathOperator{\Dif}{D \!}

which is simply wrong. The \! backspace is the cause of your problem. It's not the only part in commath that I consider wrong, so I can't recommend using the package.

A more correct definition would be

\newcommand{\Dif}{\mathop{}\!D}

that you can add to your document (with \renewcommand if you insist in using commath).