[Ingeborga]

If you're concerned about your sight, go see an Ophthalmologist if you haven't already, and get a checkup. I wouldn't recommend an Optometrist, personally. An Ophthalmologist is an MD, while a Optometrist is not, at least here in the US. While an Optometrist is good, an Ophthalmologist is better, as they provide a broader range of care.

Sound advice, especially if the deterioration is quite rapid as there could be something more serious at fault.
As has been said, AFAIK, it's generally a weakening of the muscles and/or a hardening of the cornea so it doesn't flex as easily. Perhaps cataracts could also have a similar affect?
If you're spending a lot of time looking at a monitor, reading, or doing something else where the focal distance doesn't change, it's probably a good idea to break your focus and look around so the eye muscles get a bit of a break.

Hope it's nothing serious!