[Tex/LaTex] How to avoid the changing of v to υ when writing lemmas

I'm using variables u, v, w in a lemma. However my latex compiler changes v to υ which then makes it difficult to distinguish between u and υ.

Why does it do this and how can I avoid it from happening?

I'm following the simple lemma format:

\begin{lemma}
\end{lemma}

\begin{proof}
\end{proof}


EDIT:
After David's comment, I simplified my document to point out the problem:

\documentclass{article}
\usepackage[utf8]{inputenc}

\title{s}
\author{author}
\date{November 2014}
\newtheorem{theorem}{Theorem}[section]
\newtheorem{corollary}{Corollary}[theorem]
\newtheorem{lemma}[theorem]{Lemma}

\begin{document}

\maketitle

\section{Introduction}

\begin{lemma}
Let $u$, $v$, $w$
\label{lemma4}
\end{lemma}

\end{document}