You know, after giving the situation even more thought and time, I now think I was correct in my initial feelings.
I may have held back a little in my opening post, because I wanted to be told I was wrong.. and although, admittedly, I probably jumped the gun on certain specifics, the fact remains that it seems sadly obvious now that she has been cheating on me, possibly for quite some time.
What I didn't mention is that our mutual friend is actually also my best friend's girlfriend. I have known this guy for a long time and I really trust his judgement - I have know the girl for less time, but before recently I could not imagine a reason we would have to disagree. I have the utmost respect for these people, and really no reason to doubt their judgement. Really, I have known my ex-girlfriend for a blink of an eye.
Also - the rooms in the student accomodation are really not soundproofed, and you can hear almost everything. I know this because obviously I have been there and stayed up there several times - in any case, my friend revealed to me recently that his girlfriend (who lives with my ex) has almost definitely heard my ex and her ex actually having sex on more than one occasion, but did not originally reveal this to me because she thought it would be too hurtful, and hoped the other information would be enough. She has seen him the night before, and also up there the next morning, although obviously she cannot confirm that he stayed the night it looks like it would be stupid to conclude anything else. She has been able to hear my ex's ex talking in the room on several occasions, has knocked in a totally friendly manner just wanting to hang out to see what's going on and been given weird excuses that she can't come in, the room is too messy or some other, in my view now, fairly obvious lie.
There are other things which I won't go into, but essentially this is something that has been happening over an extended period... my friends are not stupid, and as I said I have never had any reason not to doubt their judgement. If it was just one or 2 things, then yeah, maybe, it's entirely believable that things could have been misheard, it could be a misunderstanding. But for these things to be going on for several months...
As I see it there are 2 possible scenarios - one is that my best friend and his girlfriend who I also get along with very well are overly paranoid, going crazy, trying to sabotage an otherwise (I thought) quite happy relationship for unknown reasons (this is what my ex had tried to convince me of, and on a few occasions, almost succeeded). The other, is that what seems to be happening, actually, as horrible as it is, is happening. Unfortunately the first scenario is one I know is not the case.
My ex still flatly denied everything the last time I spoke to her, and speaking to people with similar experiences, it seems that she may never admit what she did to me or explain herself. I just feel numb at the moment... it is a horrible horrible feeling to know that the person you thought you knew, was actually someone else, capable of apparently complete disregard for my feelings even after all I ever tried to do was be a good partner and treat her well. I would just like to know why... but maybe she doesn't even know herself.
Obviously PTSD is not the culprit here - some people are just not good people no matter how they might initially seem.
Anyway thank you again everyone for your support and your input.