Early on in the series, when the zombie contagion was still a relatively new concept to the audience, there were a couple of attempts to depict that a person near death could actually feel him/herself turning into a walker. The most prominent depiction was with Jim, the mechanic who was bitten in late Season 1. Jim became ill fairly quickly and the group ...
It's not entirely clear, but I believe those brief flashes were meant to foreshadow Shane's rising again as a zombie. To let us, the viewers, know that Shane was infected.
Remember, this scene came before the reveal (later in the episode - thank you BCdotWEB) that:
This particular detail wasn't taken for granted at the time of Shane's death scene.