All of a sudden when I put an invoice into QB it's automatically saying paid? It's never done that before? What the heck has happened?

Thank you for posting to the Community, jekim4.

I’ll ensure to rectify your concern about an invoice that has been automatically paid in QuickBooks Online (QBO).

There are various scenarios causing the aforementioned concern in QBO. In your case, it seems that you have set your account preferences to automatically apply the credits to any open invoices to a particular customer. This will then be used or applied to it, causing your invoice to mark as paid.

To sort it out, we can turn off this feature in your company file to avoid this from happening again. Here’s how:

1. Go to the Gear icon and select Account and settings.
2. Navigate to the Advanced tab and look for the Automation section. 
3. From there, tap the pencil icon to Edit and make changes.
4. Turn off the Automatically apply credits.
5. Select Save, then Done.

You can also check out this article for more details: Create and apply adjustment notes or delayed credits in QuickBooks Online.

If you’re certain that the payment is not intended for that specific invoice. We can unlink to keep the invoice open until you receive the correct payment for it. I’ll show you how:

1. From the left navigation pane, select the Sales menu.
2. Go to the Invoices tab and look for the transaction marked as paid.
3. Tap the transaction to open the transaction summary window.
4. From there, you'll see the payment information linked to it.

You can press the Edit invoice button to modify the transaction and unlink the payment if you substantiate that the payment is not intended for that invoice. 

Once on the Invoice window, click the payment link under the Payment Status. Then, hit the Dates. You will be routed to the Receive Payment window. From there, scroll down to the bottom and press Clear Payment. To save the amendments, please click the Save and close button and select Yes to finish.

Furthermore, you can refer to this article to categorise your payments and other transactions. They will aid you with putting them into the correct accounts. Afterward, reconcile your account to ensure this is balanced.

If you have more concerns about invoices, you can always post them here for assistance. Have a good one!