There's a setting for lefties so check that. Here's a google on that.
There's also a possibility of spyware which can cause odd things like this. Remember I take your words literally. That is, you wrote "often" and not "always." If this is sporadic use Grif's advice at http://www.cnet.com/forums/discussions/how-to-remove-pup-option-603542/#post-f742c795-5881-433b-a29b-6d758efe5cd3
I've recently encountered some problems with my mouse. When I left-click, he often acts as if I've right-clicked. Furthermore the computer often acts as if I'm dragging my cursor somewhere (as if I'm keeping it clicked). My computer runs on Windows 10. I tried using another mouse or another USB-port, but it didn't work.