Apple is no longer signing certificates for iOS 5.1.1. It is impossible to downgrade to iOS 5.1.1 unless you have your blobs backed up.
There is a possible light at the end of the tunnel.
Cydia itself might have backed up your SHSH blobs for every firmware that you actually accessed Cydia from. If your device was jailbroken on 5.1.1 and you ever opened up Cydia, it probably backed up your SHSHs for you. To check, you can download TinyUmbrella, connect your iPod, select it, and go under the Advanced tab. Under that tab there should be a toggle for "Request SHSH from Cydia." Check that toggle and click Save SHSH in the top-right corner of the application. If Cydia did back-up your SHSH blobs, TinyUmbrella will download them from Cydia's servers and you'll be able to restore to the appropriate firmware as your device is to the limera1n exploit, as it has an A4 processor.
TinyUmbrella's host site:
http://thefirmwareum...a.blogspot.com/
TinyUmbrella for Windows:
http://cache.firmwar...lla-6.00.01.exe
TinyUmbrella for Mac OSX:
http://cache.firmwar....00.01a.app.zip