“Are you mad?” “Is something wrong?”

"What's wrong?" is a classic shit test. My wife does this constantly, because I get lost in my thoughts and do a shit job projecting positive body language.

Here's the thing. Your body says more than your words do. The simple answer is you're letting your body language take control of the message you're putting out every day. So you need to work on projecting a different message with both your body and what you say. The latter is pretty easy, the former not so much.

I suggest reading What Everybody is Saying to better understand "nonverbal intelligence," how to read people, and how to project with your body language differently. Until you're putting out a different story with your body language, your wife is going to continue to latch on to your stoic/resting bitch face and use it as a shit test to prove that you're butthurt just because you don't have a quick response to her demands for an answer as to what's wrong.

But you still need to pass those shit tests. The worst thing you can do is say nothin' and get upset that she is asking you the question, which is how I used to handle it.

My most common way of passing them are the classic AM or AA. If she catches me off guard, I will usually make up a humorously over-the-top thing to be upset about. What's wrong? Well, I didn't want to ruin the surprise but I bought you a pet hamster for your birthday [in six months] and figured it would survive in the closet until the big day but turns out it didn't. Have you noticed the smell? You have an imagination of your own, so use it in these scenarios to draw attention away from the interrogation and into some fanciful, distracting anecdote. And when she responds with Come on, seriously! like you know she will, just keep building on the story to make it even more ridiculous as you inch yourself closer to her to plant a kiss and grab her ass before exiting.

Edit: also the "what's wrong?" situation is so common there's a popular meme surrounding it. The joke is that women's hamsters run wild thinking something is wrong when you're off in your own head thinking about god knows what.