AITAH for questioning my fiancé after discovering he’s been hiding money from me?

I (32F) recently got engaged to my fiance (34M), and we’ve been together for four years. We’ve always been open about our finances and agreed to share everything once we’re married. We even started a joint savings account to save for our wedding and future home.

Last week, I accidentally stumbled upon a bank statement from an account I didn’t recognize while looking for some paperwork. It turns out my fiance has been putting a significant amount of money into this account every month. I was shocked because he never mentioned it to me, and I thought we were on the same page about our financial plans.

When I confronted him, he got defensive and said it was “just a backup” in case things didn’t work out between us. He insisted it was nothing to worry about and that he just wanted to be smart about his finances. But I can’t help feeling hurt and betrayed that he kept this from me, especially since we’ve been planning our future together.

Now I’m second-guessing everything, and it’s causing tension between us. He thinks I’m overreacting and making a big deal out of nothing, but I feel like this is a huge red flag.

AITAH for questioning my fiancé about the secret account he’s been hiding?