There are fake Flash Player installs all over the net. Malware hides in the downloads and you'd never know it's there until too late. Only download Flash Player updates from within System Preferences > Flash Player or from the Official Adobe Website.
Download and run Malwarebytes. Malwarebytes was developed by one of our own colleagues here in ASC. It gets rave reviews and is about the most proven anti-malware software for Mac.