Itunes deletes songs on my playlists

I am getting sick and tired of this. I just updated my Ipod Touch to version 4.1, and when it updated it it deleted my songs and playlists. I had no problem putting my songs on, but making the playlists is becoming a pain. Whenever I make a playlist on my Ipod, and then when I connect it to Itunes, Itunes deletes the playlists. When I make a playlist on Itunes, however, It stays on the Ipod. But when I add songs to it, when i connect it to Itunes it deletes my songs in the playlist. The only way to add songs to a playlist is on Itunes. Please help me with this, I had no problem with this before I updated my Ipod.