My mouse sensitivity goes crazy in CS:GO
Solution 1:
The first and most obvious suggestion - make sure your drivers are up to date. You never know just when one driver goes on the fritz, so making sure that everything is up to date and working is always a good start.
Next, as suggested, make sure the buttons aren't bound to sensitivity commands, as that would cause you chaos.
If you have access to any other mice and the problem persists, try swapping the mice around to see if that makes any changes.