So basically, I start work on a project, Set tempo, record some bass guitar, do a drum track then I minimised cubase, came back 10 mins later un minimised cubase and the tempo has slowed down! Bass recording is now a full tone lower???
This sounds as Sample Rate changes. Most probably your Audio Device changed the Sample Rate. What Audio Device do you use? Make sure your Audio Device is still set to 44.1kHz (and even before), please.