<human>: Condense the text into a few key points

Hello guys/girls,
Throw away for obvious reasons, please forgive me!

Me and my other half have been dating fairly happily for 2 years, and were seeing each other for about 9 months prior to that. We've had our fair share of issues, the main one been me catching her flirting with other guys via text. After a long discussion, I forgave her on the condition the guy is removed in all way from her life. 

After 6 months I see his name crop up via a snapchat notification on my girls phone out of the corner of my eye. That night I managed to sneak a quick look through her phone (feeling quite betrayed) and come accross this series of text messages. *removed images*

It basically says that "Sammie J" texted my girlfriend saying he/she tested 50/50 for chlamydia and that my girlfriend should get checked out. It also states he/she was tested a year ago and came back clean.

After a week of digging and finding out who this Sammie J was (My girl was still unaware at this point) I find out it is a guy called Joe C***** (censored for obvious reasons). 

I confronted her a few days ago, she adamantly denied that she cheated on me with him, claiming she fucked him 2 years ago (when we were seeing each other but before we started dating properly). Obviously I feel a little betrayed but I still can't explain why he texts her asking to get tested despite only been given the all clear a year ago unless they did the dirty since she's been with me.
I have no other evidence to prove whether they did or didn't.

She can't explain why he's listed as "Sammie J".
She claims she spoke to the former guy she flirted with about an old mutual friend being pregnant, no evidence to back that up.

Any advice Reddit? No idea if I should call it quits and make her move out of my house (rented in my name) or believe her as she seems sincere in denying cheating.
<bot>: 2 year fairly happy relationship, found damning text of girlfriend on her phone, she denies cheating on me, evidence says otherwise. What do I do?