Did you, by any chance, drop your pen or something similar?
Also, do you have any driver or software instaled for it? Because it's not recommended to use it, since it's a plug & play device. Anyways, if your pen is detecting click when you are not clicking on the tablet, maybe your pen is broken. A quick "fix" would be disable the click on the drivers, but like I said, it's not recommended to use them, because they can cause some problems.