Maybe since Jim was alive he could fight off changing for a while and since Shane was dead he couldnt fight the change.

 

We don't know how long Shane was dead.

After stabbing him in the heart, and then holding his hand as Shane realized he was dying, Rick may have sat with Shane for a few moments or even an hour - they were BFFs since high school. Shane's gunshot may have caught the attention of Carl, who walked over in the direction of where he thought it came from. It would make no sense for Carl to be wandering around at night...even though at that point everyone thought the farm was safe.

It could have been an hour until Shane came back....depending on how long Rick sat with him.

 

I think it was at least a few hours before Amy came back. She was killed at night, and it looked like it was the middle of the day when she turned.

 

Good point. (And I didn't realize this thread is more than a year old. Ooopsie)

Some of the other points made sense; Shane was full of rage (flipping out in the barn, killing Randal) before he died, and Jim seemed like he was trying to wake up and avoid the visions.

Ed was certainly full of rage, but didn't turn..... hmm...

 

I sure the flashes were Rick thinking about what was about to happen. He knew that Shane was going to turn and was pacing around waiting for it, the flashing of walkers clouding his already frantic thoughts (just had to kill your friend, gotta kill him again, etc). And I think it adds to the drama for the viewers too, since this is the first time we see someone turn from no walker bites (we didn't get to see Randall turn even tho he was technically the first person we saw).
Rick had his gun taken away, so he couldn't just shoot Shane in the head (when he was alive), and I think he wanted to see for himself if it was true that no matter what, we all turn, and what better opportunity than when you're alone with a body you just killed.

 

ummm he was wandering around alone at night, at that point Carl was constantly sneaking out and roaming around hence all the screams about Lori being a lousy mother.

Personally I took the whole flash thing to be random firing of Synapses as the Virus spread up into the Brain before it shutting it down.