Ipod sync error will erase all on ipod!

Hello,
Got the 8gb Nano iPod...downloaded iTunes to my computer. I get the message that this iPod is synced to another iTunes library and if I continue to sync, all items on my iPod will be erased. When iTunes is opened, I click on device and it opens the summary for my iPod but can't get any music to list that is already on my iPod. I've only used the one computer and the iTunes I downloaded on that computer. Any ideas?