It is just a very common situation. I can do this too. What you are doing is accomodating focus of your vision. The muscle in our eyes can contract or relax to change the shape of lens according to the oder from our brain. With the shape change of lens, its curvature is changed too, which lead to vision focus changed. If you know camera, you must know what I am explainning. Eyes are working the same way exactly with camera. With a camera, if you want to take a good and clear picture, you need to adjust the focal length. Taking pictures at different distance requires different focal length. If you are looking at something up close, yet you send orders from your brain to eye muscles, asking them to adjust the focus on to far distance, then your vision is blurred. Vise versa. No need to worry about it.