Sounds like you have something running that shouldn't be: Virus, malware, or even spyware! Or the drive is having problems.
At this point I would make a backup of your data files and anything important. You'll need to make a bootable drive to run Disk Utility to make sure you don't have a bad drive. If the drive shows its OK, then you'll need to wipe the drive down and install a fresh copy of the OS.
Once you get to this point you'll need to be careful you don't re-infect your system. Maybe it's a good time to get a good antivirus app and check your backup drive to make sure the files are clean before copying them back on. I would also recommend you make sure the apps you install are from the original source (App Store or original creator of the app. Not off of any free site as some of these are infected.
Lastly make sure your drive has some free space. SSD's need it more so than HD's. My rule of thumb is to leave 1/3 of the drive free if the drive is less than or equal to 256 GB on 512 GB and larger you can reserve 1/4. If your systems RAM is limited to 8 GB you'll need the extra space for virtual RAM more so.