Your extension did not give you extra time to pay your tax due. An extension only gives you extra time to get your documents prepared. You were supposed to pay your estimated 2018 tax due by April 15, 2019, so your payment is now late and is subject to interest/penalties.
When you requested your extension you were supposed to pay your estimated tax due. Did you enter the payment you made into your tax return? Go to Federal>Deductions and Credits>Estimates and Other Taxes Paid>Other Income Taxes
When you prepare your 2018 tax return the amount on line 22 of your Form 1040 will show you the tax due.