Baby-sitting a child with an aversion to diaper changing?

The other day when I was baby-sitting a 1 year old he needed a diaper changing badly, so I started changing his diaper of course. As soon as i started unzipping his pj's he started wailing like i had never seen this (normally laid back) child do before. He tried to hold his diaper on and when i asked his 3 year old brother if this was normal he said no; and the child only looked at us and continued wailing and asking for his mommy when we asked what was wrong. Was this just because being tired or some other reason? And should I have not changed his daiper despite the risk of diaper rash?