Getting mass text response without sending mass text?!

I recently upgraded to an iPhone XR a few days ago. Same phone number I’ve had for many years. Today, I started getting texts with just the word ‘stop’. It was a group of 19 on this text, and multiple were sending it. I do not know the people, don’t recognize the phone numbers. There is no initial text that shows what they are responding to. It looks like my account sent a mass text with the statement to reply ‘stop’ if you no longer want to receive texts. But I don’t see it. Just a bunch of ‘stop’, one person said to not use their number again. I called ATT support, which told me to leave the group text. There is no option to leave the group text. She then said to block them. I said I was worried someone is spoofing my number, or someone is accessing it. She said my number could’ve belonged to a spammer before me, but I’ve had this number for many many years. She then said she couldn’t help anymore and that no one has access to my number. I’m still a bit concerned. I responded on the text string asking what was going on, but no one responded to me. Then I got a ‘stop’ response. I would really like to know what’s going on.