They were messages sent from my phone without my knowledge. How was that possible?

Hello ~ Do you share your iPhone , passcode or Apple ID with anyone? Has you iPhone been unlocked and in the hands of someone else? You may want to take a little here by clicking on the following helpful link…

If you think your Apple ID has been compromised - Apple Support

Also change your iPhone passcode and keep this information to yourself…

Set a passcode on iPhone - Apple Support

~Katana-San~

They may not have been sent from your phone, they could have went out of other Apple devices like: a watch or an iPad.

You can undo message forwarding by going here in your phone: Settings, >Messages, >Text Message Forwarding... See if there are other devices showing and if so, is the toggle on or off?

You'd also need to check those settings in your iPad if you have one.

After turning the forwarding off, I predict that you might be back here in 6 months with a question of, "Why aren't my messages syncing across my other devices?"

So... Maybe the best thing to do would be to set up Face ID on all of your devices and also create a new unlock pin if you suspect that someone in your household knows your current one.

I hope that this will help you out :-)