Last time I logged on, I owed taxes, which I couldn't afford to pay... so I filed an extension. When I hopped back on now to file my taxes, I don't owe anymore. How?

Did you make a payment with the extension?  The extension gives you more time to file the return.   The tax due was still due April 18.   Your return is probably assuming you made the payment.

So be sure an extension payment doesn't automatically show up on your return! Check 1040 line 31 which comes from schedule 3 line 10.  You may need to delete it.   See this Extension FAQ for how to get there.  Then you can change to amount to zero.

https://ttlc.intuit.com/community/tax-payments/help/how-do-i-get-credit-for-tax-payments-i-made-with-my-extension/00/26817

did you enter an amount for the extension and just not pay it?  if so, Turbotax may have included that as an actual payment. if so it will show up on schedule 3 line 10. you'll need to go into your return to delete this "payment". if you did not fairly accurately indicate the balance due on the extension, the IRS can invalidate it.  in this case, we have no way of knowing why your balance due dropped to zero. we can't see your return. 

Are you sure you signed in with the exact same user ID that you used when you prepared the tax return and filed the extension? People often have more than one TurboTax user ID, sometimes without realizing it.