There is an event that decreases all influence, and that may have happened in your game.
There is also one that increases everyone's influence. One usually follows the other after a certain number of weeks or months.
Not much you can do about it, except to plant more influence bases (which may make some civs declare war on you) or build some of the special buildings that increase influence on a global scale. You can also get some influence resources and load them up with mining modules.