One contact lens keeps blurring

I've been wearing the same brand of contacts for a few years now, but a few weeks ago my left eye was seeing blurry with halos around lights so I put in a new pair.  The next day, the left eye contact was blurry again after wearing it for a few hours.  I cleaned it and put it back in, but it was still blurry so I opened a new contact lens and put it in.  It was fine until I wore it for a few hours the next day--it got blurry again.  
What could be the problem with that eye that it keeps ruining the contact lens?  A list of differential diagnoses would be appreciated :-))

[John C Hagan III, MD, FACS, FAAO]

90% contact lens problem  10% other eye problem.

Stop wearing your contacts and return to the fitter for a re-evaluation.

JCH MD