You should firstly check whether the things that you focus on are clear. If they are not clean with gray spots, your eyes are OK and have no problems. If not, you must check your eyes whether they are the floaters in your eyes. Your eyes may get infection that makes you see things in blurry. You'd better go to see the doctor and have a check.
Of course, you need to provide more detailed information, then it can be sure what it is. Normally, if it happens occasionally or in a short time, that is to say, you just need to take a break, then you will benefit a lot ,naturally, the symptoms can eliminate. Of course, if you are not in this kind of condition, maybe you should be more cautious. It is possibly caused by vitreous opacity, if it goes on , you may have much trouble, so you had better do fundus examination to make it clear. At the same time, it should be pointed out that, you have to pay attention to your way of life, to developing a good work and rest time and the eye habits.