Eric70, it sounds as if you may have been spoofed. Basically, a third party app can be downloaded to mask a number and have it appear as a call or text may have come from a different number. It's a concernindustry-widee that can happen on any number, wireless or landline.
Here are a few links with more details:
How can this happen? If there a way to prove I didn't send it? It is not the first time or has happened.
Does this text show up in your text message logs or in the web access to Message+?