This could be occurring for a number of reasons, but it is my advice that you should not seek to have this diagnosed or remedied over the internet. Without evaluating you in person, it is impossible to determine what is causing this. It could be a sighting problem, stance problem, line up problem, breathing problem, or even a cue problem. You have noticed the result, but without having a certified instructor evaluate you to find out the causes of the result, the help you recieve here will be of no use to you. Or, the advice you get here could be incorrect, which will add to your frustration when your problem grows worse. Find a qualified instructor to evaluate your fundamentals and in the long run you will save yourself time, money, and heartache.
Good Luck