It sounds like your repo data has gone stale, corrupt, or something has changed since you last updated.
yum clean all and then try
yum update again and see if that has resolved your issue.
If it's still not working check the
epel.repo file which you should find located at
/etc/yum.repos.d/epel.repo you should expect to see something like this (extracted from my file)
name=Extra Packages for Enterprise Linux 6 - $basearch