un intentional key/pitch shirt on audio

[Debmass]

i am having a problem that seems to either be in my head or some sort of software/hardware glitch.

i am recording a song using reason as an instrument track for the drums,
i am using my digimax fs as my preamp for my bass.

, tuning my bass and my guitar before hand, i lay down my bass track.

the session runs a bit long, and i have to shut down and go to sleep.
this morning, i attempt to add the guitars, but the bass is about a step to a half a step higher than the night before.
the bass is still in tune they way it was when i recorded the track.

has protools somehow compressed my track?