after login, the screen becomes distorted with horizontal stripes

I have a Macbook Pro (Retina, 13-inch, late 2013, running on OSX 10.9.4) and since about one month the screen immediately after login becomes distorted having grey background and horizontal stripes. See the image included. The distorted screen lasts for some seconds, and the computer continues to start up normally afterwards.

I am not sure if I need to worry about this or not. Though it is not normal. Would someone have an idea what causes this problem? I did the Apple Diagnostics test, and it showed no problem.