I "erased all content and settings", but did not sign out of iCloud before selling my iPhone?

No, once you "Erase all content and settings" your iPhone is basically back to factory settings and the new owner will not be able to sync with or sign in to your iCloud account.

To expound on some of the answers - there's two different things at play here

• Signing out of your Apple ID
• Turning off "Find my iPhone"

Although it sounds innocuous, turning off "Find my iPhone" is the important part because it turns off Activation Lock, which if not turned off prevents the phone from being able to be reused.

With Activation Lock turned on (so, with Find my iPhone turned on) if someone were to steal your iPhone and then try to nuke it to factory settings, during the setup process it would say it can't do anything until the Apple ID associated with the Activation Lock enters their password. The thief isn't going to know this so the phone is useless to them or anyone else now. When word of this got around the criminal community thefts of iPhones dropped dramatically.

But signing out of your Apple ID itself isn't necessary. Once Activation Lock is turned off, resetting your iPhone is like reformatting your hard drive.

Also I think iOS warns you if you try and reset the device before signing out of Find my iPhone.

If you didn't sign out of Find my iPhone, whoever you sold it to may be contacting you soon because they can't use the phone without it signed out, but if you vaguely remember turning it off and no one has contacted you, you're probably good to go.