Different computers

Unless you enabled disk use on your iPod and specifically transfered the songs, your wife's Mac should not have just magically obtained your songs when you plugged it into your computer. In fact, it should have told you that the iPod was associated with another computer and asked if you if you'd like to use the library on the Mac (your wife's music) instead. If you said yes, it would have deleted all of your music and loaded her music onto your iPod.

Here is an article on how to move your music to her computer if you should choose to do so.