What could cause my eyes to change color?

Some eye drops can make the iris color darker.  There are conditions that can make one eye become lighter due to inflammation or pigment loss.  Conditions that will cause both eyes to become lighter would include conditions of the cornea that might give the appearance of lighter eyes but this would very unusual without other symptoms.  It is also possible that this is a condition that is glaucoma related called pigment dispersion where the irises lose pigment.  However, I would guess that this is likely to not be anything serious.  However, you certainly should see an Eye M.D. to be sure.