Financial infidelity is committed when you intentionally hide your spending from your husband because you're afraid of what he'll say about it. Have you paid cash for items that you didn't want to appear on your joint credit card bill? Or maybe you've slyly disposed of shopping bags or receipts?
If you've agreed to combine finances with your husband, then chances are your tactics won't go forever unnoticed. Sure, it's great to treat yourself once in a while, but it's best to be honest about it to avoid any money arguments pitted around your lie.