Reformat and clean reinstall your PC and any/all other access points (phone, tablet). Any home WiFi router also factory reset and set-up with different credentials, secure config. Once so is done (to avoid a keylogger or other spyware), then and only then change e-mail account credentials to a good strong password, Delta account credentials, all other bank and online account credentials, etc.

Reformat and clean install is a major hassle, I know, but in your case it pretty much has to be done, as there is a keylogger or some other spyware on your PC, phone, tablet or elsewhere, so not just your DL account but your banking and other accounts are likely compromised also.