Check your video display settings.
When this happened to me, it was because I had put everything onto a new laptop, and then, noticing that the very nice display screen was too bright for my eyes, started frigging with the video display controls. This included the brightness, contrast, colour and gamma settings. I got the display to a state that seemed to suit my eyes/perception, and then noticed that images I had in my Picasa database and that I had previously tuned to perfection on my old laptop+display did not look quite right now on the new and adjusted display! (Not surprising really when I thought about it.)
What you see on your display is what you perceive from the display after the computer graphics have translated the image for you according to video display settings. If the display or its settings have been changed somehow, then the same images that appeared fine before may well look different/odd now.
I reconciled the variability in my case with the fact that any published images had been adjusted in good faith and that it was too tedious to go back and readjust everything.
Belts and braces recommended: I could always go back to the originals in Picasa if I wanted, as I rarely delete or overwrite important image originals.