Why to the check numbers in my register change on their own?

I’ll be with you through the process of making sure it follows your check number sequence, LynneM39.

QuickBooks Online automatically places the transaction numbers unless you turn on the Custom Transaction number feature and manually entered one in. This option allows you to assign one check number and the rest of the sequence follows.

You can verify if the feature is enabled on the Account and Settings page. Here’s how:

1. Click the Gear icon and choose Account and Settings.
   
2. Tap the Sales in the left navigation pane.
3. From the Sales form content section.
4. Check the box next to Custom transaction numbers.
   
5. Hit Save and then Done.

For more details, refer to this article: How to change the invoice number on sales forms and other transactions.

Once done, you can start reconciling your account by following the steps in this article: Reconcile an account in QuickBooks Online. 

If you have any other questions, please let me know by adding a comment below. Cheers for more success!