UPDATE: I (42f) receives a text saying my (46m) fiance has a girlfriend

My last post got locked due to my lack of karma, or something like that. I'm not sure how to edit my original post, so I'm updating this way. I also don't know how to link to my original from mobile.

I didn't say anything to Caleb until I got home. I think I had a suspicion that the texts were not legit, but I needed outside perception. I walked in the door and told him that we needed to talk, and showed him the texts. He legitimately looked confused, then I showed him my call log. He was blown away, then immediately offered me his phone. I looked through it, and found nothing. No hidden apps, nothing in the trash or call logs, nothing in his gallery, nothing in his social media.

I had texted the "person" earlier that day (0800) asking if they had any proof, by way of screenshots, dates they were together, or photos. I wasn't salty in the messages at all, and I stated that we were supposed to get married in August. When I didnt hear back for about 8 hours, I sent another text saying Iwas calling BS, and that I didn't know what their endgame was, but I wasn't going to fall for it. About 1945, they got back to me, "I had no idea you guys were engaged he said you lived two hours away. Tell him to lose my number". I responded with "Can you send me some screenshots? When did you see him last? Do you have any pics together? He is denying everything. I'd appreciate it". About 5 minutes later they came back with "Honestly I have no interest in breaking up a marriage and I should have minded my own business. We slept together a few times and it really wasn’t a big deal I apologize for any issues i’ve caused". That really got to me, so, my response was "You helped, but it was he's choice. I would appreciate having proof" then I sent one right after this one saying "And you should tell him yourself"

After about 20 minutes I texted that this was BS again. They responded almost right away and sent this final message "I’m at work. Can you stop? Stay with Caleb, don’t, do whatever. I wanted to make you aware of something that involved you. I thought i’d be doing you a favor but you continuously accusing me of making up bullshit and texting me four times after I don’t respond for more than 10 minutes is making me think you should just keep him. Unless you’ve fucked over someone, im not sure why you’d assume someone is fucking with you. Also I don’t see what I would benefit from this considering he’s engaged and lied to me about it and I blocked his number already. Have a good one". The tone in this one changes, and something said in a previous message changes my suspicions.

1. I am originally from a town about 2 hours from where I live now, but I have lived here for almost 4 years
2. Caleb doesn't tell people where I am originally from. The city/metro area I live in has about 1.5 million people, and it is never asked.
3. I was in an abusive relationship up until about 9 months before I moved here, and when we split he displayed very stalker-ish behaviors. The ex even went so far as to use a number spoofing service to contact me after I blocked him.
4. Caleb gave up his phone easily, and without hesitation. His non-verbals showed genuine confusion regarding the whole situation.

The conclusion that I drew was that this is someone from MY past trying to either, destroy my relationship, or test how strong it is. I'm thinking that whoever it is discovered that I'm in a relationship somehow, and wanted to shake it to see what would fall out. We ARE still together, and are still getting married.