A few years ago developed something similar and it is still there. I't's like a little speck (of dust or similar) which "floats" across my vision. I consulted an optician at the time and he told me it was a "floater". It is apparently not dangerous and is something i have grown used to. I hardly ever notice it at all now. If you are concerned it is worth seeing an optician or doctor to set your mind at ease, because I can only tell you about my own experience.