I’ve been in your position (or at least a similar one).
It might sound a bit “loose Women” but why should you put up with this sort of thing?
There are women about who won’t behave like this they are not all the same. You don’t trust her so just have it out its better to know at least.
This other bloke is incidental a “bit of rough” not a long term thing from her POV but probably enough of an indication of how she truely values your relationship…
Often women want out but somehow need for you to be the “bad guy” and end it the phone snooping confrontation might be the trigger for that. again at least you’ll know one way or t’other. By the way did you check her sent messages or just the inbox? often sheds more light can be a tough read so prepare youself… If it turns out she’s been doing the dirty don’t engage in a long shouting match pack a bag and leave quietly give it 48hrs for your head to clear after. no booze no texting her get yourself some space and time to think.
Good luck with it all.