Why does the subsection appear in the header before it does in the document?

I am using this code

pagestyle{fancy}
fancyhf{}
renewcommand{headrulewidth}{0.5pt}
renewcommand{footrulewidth}{0.5pt}
lhead{textit{leftmark}}
rhead{textit{rightmark}}
rfoot{textit{Page thepage}}

renewcommand{sectionmark}[1]{xdefleftmark{thesectionquad#1}xdefrightmark{}}
renewcommand{subsectionmark}[1]{xdefrightmark{thesubsectionquad#1}}

to print the subsection title in the header. However, I have the issue that if the subsection is printed at the top of the page, then it shows in the header of the previous page. This is shown below.

This is Section 1.1, but the header shows Section 1.2. How do I fix this? My current solution is to do the following.

begin{Def}
Let ${M_i:i in {1,ldots ,k} }$ be a family of submodules of an $R$-module $M$. Then $M$ is the textit{direct sum} of $M_1,ldots ,M_k$ if
indent  i) $M=M_1oplus ldots oplus M_kcoloneqq { m_1+ldots +m_k: m_i in M_i}$; 
indent  ii) $M_i cap M_j = emptyset $ for all $i neq j$.
end{Def} 

newpage

So I just add a newpage command at the end of the page and that fixes the issue.

renewcommand{sectionmark}[1]{markboth{thesectionquad#1}{}}
renewcommand{subsectionmark}[1]{markright{thesubsectionquad#1}}