You may need to get your IRS transcript and compare the tax return you filed with your transcript. If the IRS reduced or denied a deduction or credit, resulting in more tax owed, it should show on the transcript.
Or, the IRS might not have received the payment from the credit card processor, or they might have sent the notice before receiving payment. Your transcript will also show if a payment was applied to your account.
https://www.irs.gov/individuals/understanding-your-cp14-notice
https://www.irs.gov/individuals/get-transcript
