The Guy at my Girlfriend's work... Should I be concerned?

So I have a fairly new relationship with a girl (4 months) that has been going great so far. At least until now...

My girlfriend's dryer at her place hasn't been working right for a while. A coworker of hers, who I believe is around 35 to 40 years old, told her she needed to replace a small part to fix it. She then ordered the part and was going to wait and see if he could come replace it at some point. She then told me that apparently his girlfriend is a bit nuts and won't let him come to her house because his girlfriend doesn't like her. I asked why and she said they were all hanging out once a few years back and apparently this guy told my girlfriend that his girlfriend thought she was giving her the eye or a stare down you might call it. I guess his girlfriend doesn't trust him to not cheat. Then my girlfriend said and he is way older anyway, I don't get what she was all upset about. I found it odd I was even hearing this all in the first place.

So a few days later she texts me while we both are at work and says, hey do you want to try to replace the part with me? I said sure, i'll give it a shot (i'm pretty good with electronics/fixing things). So I started looking up the model with the schematics/etc to learn more about it. I texted her and asked her for some more information on her dryer and she responded, you aren't working on it now are you? I said no, just researching, she said ok, good.

Anyway we meet at her house and I was there first. She was all mad right away when she came in thinking I was working on it apparently. This is really odd behavior as well because she hasn't been mad at me at all yet. So during my research I found out that the part this guy said to replace is the wrong one. I gently broke the news and she got even more mad (that probably wasn't the greatest plan I realize... but i had to be done). She was clearly even more upset and said well I don't think he (the guy from work) would lie to me. I said I don't think so either, I'm just pretty sure this isn't going to fix it. I even defended the guy saying that he hadn't physically looked at it and probably just wasn't aware of all the symptoms/etc. I asked her if she was mad at me and she said yeah a bit. It seemed like she was defending him pretty heavily. Anyway the rest of the night was rough for a while.

Later that night I was working on some stuff ( i work on phones) and i needed to test some calls from multiple phones. I used her phone and saw a text from this guy. I know I shouldn't have, but I looked at it. What I read really disturbed me, but I'm questioning whether it should have or not.

Essentially the few day conversation went like this (just meaning when they talked about the dryer first until the current day I saw the conversation on the phone) : She asked him if he could look at her dryer or possibly fix it. He said well we both know where that will go, haha. She said Noo... That's naughty. Then the conversation turned away from that stuff and just to fixing the dryer for the rest of that day. The next day of the texting she said to him hey, thanks for doing this, I can pay you if you like or give you whatever you want. He said no hun, that isn't nessassary, i'll do it for free. Then she said I'd give you what you really want if I could, haha. He replied with a picture that said paper beats rock, rock beats scissors, but nothing beats a blowjob... She said, yep thats very true. That was the end of the conversations I saw.

So when it call comes down to it, I have a feeling they probably have a past, which doesn't bother me. What does bother me is the weird flirting and possibly opening up that past again. I know she said she couldn't and all, but I've seen that kind of crap 100 times with girls, they say no, no, no and then it is yes. Also she sent me that picture that day and we laughed about it (i didn't know this dude had sent it to her at that point). I asked her after I saw the texts about where she got that picture cause it was funny and she said I don't know. I was like, what do you mean? She said she couldn't remember... Also I found out she deleted that whole message chain from her phone.

I'm just not sure how to feel about this. I'm sure she hasn't cheated on me with me since we have been together or with anyone else for that matter. She has been a great girlfriend so far. Supportive, kind, affectionate, compassionate, all the things you would want. I should mention my last relationship, which wasn't too long ago ended really bad. That girlfriend cheated on me and we broke up. We were even living together... So I'm not sure whether I'm blowing this out of the water due to the past or if I should feel betrayed here. On one hand, she basically said no, but then she came back and said I would if I could. That is the party that gets me. I can deal with the scummy dude acting out because that is going to happen no matter what, but I don't feel she should even hint towards anything sexual with another guy period end of story.

Thoughts? Thanks!