Contacts naturally dry out in your eyes, hence the blurriness (and if you have toric lenses like I do, God help you if it gets rotated even a little bit).
I'd recommend having a thing of eyedrops/solution and get your lenses wet at lunch or something, wherever your midday is. If it keeps happening, refresh the lenses a few more times each day. And I'd talk to my optometrist about that, since you're supposed to get at least 8 hours per wear of vision, though I stretch mine to 16 no problem. =P
...and I also wear my one-month pair for a year when I can. *cough*