when a person focuses at near objects (e.g. computer screen) for a
prolonged period of time, they may develop increased ciliary muscle
tone.  (the ciliary muscle is inside your eye and attached to your
lens.  its activity causes the lens to change its shape so objects at
near are in focus instead of far away).  the increased tone of the
muscle, i.e. its inability to relax completely, may cause your distance
vision to be blurry for a short time.  after you quit working at near
for excessive periods and resume a more normal visual pattern the
muscle relaxes to its normal state and your distance vision appears
better.  this is commonplace and is probably what you are noticing.
this effect is called a lot of things, and I use the term
"pseudomyopia" to describe it.  it probably isn't related to lenticular
astigmatism.

There are several ways to get similar symptoms, but increasing lenticular
astigmatism isn't the most likely. And there's no evidence that computer
screens cause any more of these effects than printed paper.

It's hard to get the true explanation without having an eye exam, with
cycloplegic refraction and examination of your cornea and tear function.

Sometimes reading glasses help by reducing the amount of effort you need to
work at near.

Sometimes artificial tears solve the problem, or using a humidifier.

Sometimes changing the position or distance of the monitor works.

Sometimes your symptoms just mean you're starting to get nearsighted.

-MT