slow playback

[Roshan]

okay, so i'm really new to the protools world, so bear with me.
last weekend our band was recorded through a digi 002 rack system.
the files were transferred to my hard drive, and everything seemed fine.
but when i play back the session, the first minute and a half sound fine, but then everything slows down.
the singer's voice gets deeper, and the pitch gets lower, not to mention the tempo of the song slows.
has anyone experienced this before, and is there a way for me to speed everything up?
any help would be appreciated.
thanks