I did not quite get what you meant. Is it everytime you click the left mouse button that you get the context menu or just once in a few clicks?
If its everytime;
Then someone could be playing a prank on you or your mouse settings were changed. Go to control panel > hardware > mouse and see which 'button configuration' you have. If its 'left-handed' then just switch it back to 'right-handed'
If its once in a few clicks;
Its a common problem with optic mice and its just coz the mouse is dirty. Try cleaning it with a soft brush(including the ridges between the buttons) and blow into it from under a couple of times. If the problem persists, then maybe the mouse is dead coz of too any clicks

This is a common symptom of gamers' mice

)
Good luck!
if that helped you, give me some reputation
