When typing your unencrypted private key by hand, you need to be ultra careful that you type in correctly. Mistyping a single number or letter will access a different wallet, and make it appear that your address has changed.
Alternatively you could use the QR code on the paper wallet, or save the QR code when you first generate the wallet, in order to decrease the likelihood of this happening.
A lot of times when you first mistyped it and see a different wallet / no balance your adrenaline starts rushing and you start freaking out which only makes things worse. Ive gotten emails like, "everytime I try to access my wallet it spits out a brand new address". Take a deep breath. Retype it, carefully. It'll be okay.