Digital BW, The Print

Does anyone here have a Sony Artisan? I am having problems with images in Photoshop 
that I have saved...on any random day when I open them they are all darker! This has 
happened several times and it's incredibly frustrating after I have worked so hard to get 
them to look a certain way. The monitor stays calibrated for 30 days, then you must re-
calibrate it. I have one day left...and while last night the images all looked normal, tonight 
they are much darker...could that be the problem?

Can anyone explain this?

Thank you,
Sarah