iPod touch log out

[nicspotify]

Could you please help me understand why Spotify on my iPod touch occasionally just logs me out when I am in offline areas, thus cannot log back in using either Facebook or Spotify details? It doesn't do it every time, but a few times a week.. 

When I leave Wi-Fi areas I always switch Spotify on to the Offline Mode, which should effectively keep me with my music for 29 days, as it says, however, it will often only be 5 mins... 

Thank you!