Well it sounds like most of us really can't! She's probably just having a moment of irrationality as we all do from time to time. There's probably something else going on but I doubt it's worth it to try to figure it out.

Did she say why she was upset? Is she worried she's eating too many? Just that she thought it was a prank and not nice?

I don't think you're an AH, but it does sound like your wife might be insecure about how much chocolate she's been eating. I'm picturing her telling herself "I'll let myself eat this much but then I'm back on a diet!" Or something to that effect. Then come to learn you've been foiling her plan in the background. Which is also something someone who is insecure about stuff might not want to say: that they are on a diet or worried about eating too much.

All just a bunch of assuming.