This phenomenon is quite normal and do not cause serious probems. This symptom is called floaters. Floaters often show in various size, shap consistency within your eyes. You'lll notice that htese spots can be found especially when you peer at a bright, clear sky or a screen. Sometimes you try to catch them, however you can't. These troublemaker never seem to stay still. To put is simply, most cause of floaters is shrinkage of the vitreous humour. The gel-like breaks down into fibrils which are the floaters.
Frankly speaking, I also have such experience, but not very often. So I just ignore it. But if you see a shower of floaters and spots, sometimes accompanied by light flashes, you should seek medical attention immediately.
