I'd say a little YTA...

I think you're looking at this from the wrong angle.

You know you have a neighbor watching your kid. She doesn't. She just sees a little kid walking five minutes all by himself.

He may be responsible, but there are a lot of creeps out there. Even though the odds are in your favor that he'll be just fine, any number of horrible things could happen no matter how responsible your son is.

While you're upset about being judged, and about what she may have said to your kid, it sounds like that's what this woman is really worried about.

Now, if she really did say that to your kid, then she's judging you unfairly. In that case I'd change my vote. But you really ought to just talk to her first before making the same mistake that she might have made. Until then I think it's premature to judge her too harshly.

You can definitely ask her about what she said in order to clarify what she meant, and be sure your son didn't misunderstand. You can tell her about your surgery and explain that you had a neighbor watching your son from her house.

Personally, I'd be grateful if someone else was trying to look out for my kid's safety. Hell, maybe she'd be completely happy to help you out and would be a good second person to have looking out for your son on his way to the bus stop.

So basically, I think you would be the asshole if you judged her prematurely and approached her as if she's some nosy bitch that should mind her own business, as opposed to approaching her calmly as if she's just a concerned parent that's concerned about your kid's safety.