When I am "cycling" I get hallucinations which apear out of the tails of my eyes. When I look, they disappear, although they might be very detailed in my periphery. Most likely, you are geting "floaters" which are adnormalities in the fluid in your eyes. That can mean lots of things, and since you haven't really described what you are seeing, I can't tell you which you are seeing.
Hugggsssss, and whatever they are, tell a doctor if you are worried. That is my chant.