Is it possible that you were pressing the Shift key, which forces a line from the last brush touchdown? Have you tried saving the image as a JPG, and reopening in another application? (that would rule in or out artefacts caused by video card driver) The red lines look like you might be in Quick Mask mode, although that wouldn't have any bearing on the issue that I can think of.
I'd be inclined to check that your video card driver is up to date from the card maker's website.