The messages logfile can get pretty big, and depending upon how often it rolls, interface changes might not be there. You can create your own logs, so here's what we do to create an log file just for interface changes:
set system syslog file interface-changes any any
set system syslog file interface-changes match SNMP_TRAP_LINK
The last line says to only match log entries that have the string SNMP_TRAP_LINK, which will catch the interface ups and downs.
Now you can simply do:
show log interface-changes
Rob