The whole Rick is a zombie sort of, doesnt really fit with the character though.
He's not necessarily thinking like a zombie. I think this is some wacky medical thing that's yet to be explained, but will clarify a lot about what's been going on.
If it's not related to the virus doing something weird to Rick, then what could the whisper-mystery be?
My initial instinct would be that it's simply, that everyone has the Zombie bug already, and no matter how you die, you'll become a Zombie, wether bitten, gun shot, natural causes etc, since that is common in Zombie lore these days.
problem is, by Rick not sharing this, he could put others at risk, for example Lori cuddling Carl as he's dying from the gunshot wound, if he had died in her arms, he could've turned and bit her. Would he withold that information? Is it OK for him to withold that information, since people don't turn immediately upon death in this show's Universe? Or does this rule out this possibility?
I don't think that could be the answer, just because it's not actually shocking enough. It sort of makes sense that everyone would become a zombie now if they die, and is it really that big a deal?
If someone were to die from anything other than a zombie bite, you just need to make sure to shoot them thru the head before they revive. People would get used to this pretty quickly, after all, the person is already dead.
And Rick certainly would not withhold the information. It's plausible that it's true, and dangerous enough that everyone needs to know about it.
Whatever it was, it was so shocking and crazy that he doesn't even believe it. He thinks the guy was lying to him because he was sort of malicious, wasn't he? Rick having died and revived, yet not zombified, qualifies as that sort of thing.
It seems more likely to me that if he'd been told by Captain CDC that he had the zombie bug, he would have either stayed in there to be incinerated, or headed as far away as possible from the group. And i doubt he would have risked infecting Carl through blood transfusions.
If he doesn't believe it - and it seems pretty unbelievable even in context with everything - then he'd behave exactly as he has. He had to give Carl infusions to save his life, so if that had killed him, well, Carl was going to die anyway.