You are probably infected with Spyware or Adware, two forms of Malware. This symptom should be addressed immediately as these infections often open the door to infections that cause more severe problems.
Scan your computer for malware. I recommend the Malwarebytes program for this purpose. This program is included on the AntiDisk, sold on Ebay.
It is easy to use,a free version is available and is not limited for cleaning and removal.