Hotbar gradient effect

The attatched image was not photoshopped. I press F11 to fullscreen on multiplayer, but when I pressed F5, the hotbar blacked out and gradient - pink effect came up. After pressing F2 to take a screenshot, I disabled full screen, and it was back to normal. Also, I don't know if this made a difference, but a lag spike occured, and I had the F3 menu open. GIU was on normal, not auto.