Typically, that's a sign of malware on your computer. Do you run anything that scans and detects malware like MalwareBytes or Ad-Aware?
If you don't, first thing I would do is download MalwareBytes and fun a full scan. You can find the download link here:
Malwarebytes Anti-Malware - Reviews and free Malwarebytes Anti-Malware downloads at Download.com
Just click the Green Download now button.