Thanks for all your replies.
Just a bit of backstory and context. When we first started dating, perhaps lets say a few weeks in- she told me she kissed her best friend (a girl) whilst on a night out. That night in question, I had recieved lots of missed calls from her, and I had a panicky voicemail from her and numerous texts to the tune of "call me ASAP". I didn't actually see any of this till following morning.
To cut the story short: I rang her up the next day- she had explained to me rather abruptly that she'd 'got off and kissed' her best friend. She said she'd felt incredibly guilty and burst into tears immediately afterwards and wanted to tell me asap.
My immediate reaction was one of disgust and a huge disappointment in my heart. However, I admired her respect for me in that she told me almost soon after the incident. Notice, I didn't waiver the fact she had 'disrespected' me by kissing somebody else.
I probed for answers, although she refused to tell me which 'friend'. I knew she was bisexual, but also that her girly friends kissed each other all the time. But because I knew of her sexuality it played on my mind that she may have actually do this lustfully with sexual intent.
She explained that her and said mate did that sort of thing alot before me, and that she just didn't think (completely forgot and was drunken).
For some reason, I looked past it, after several drinks and the notion that she'd 'fessed' up immediately. It was a hard truth to swallow, and I had that horrible tight feeling in my throat/chest. Part of me said 'kick her to the curb' the other part told me 'I've only just started going out with this girl, shes already strayed-- but do I forgive her due to her sincerity?'.
Anyways- we moved forward, for a while she was very transparent and always honest with me. I'd press her about it for ages and ages.
I'd almost forgotten about it (nearly two years ago); but it always has a way of creeping in when her routine is out of the ordinary, and because I now live other side of country its even harder.
We're engaged, our relationship has changed, we are much closer and our connection is deeper. But 'once bitten, twice shy' as they say. There is always that query of ' what if'. My mind is active all the time....
Also- she had once left her Facebook open and I saw that she'd been communicating with some guy. He'd been saying to her: "Oh hey babes. How're you? Oh your close to me, we should meet up amazee-ballz"...communicating in a very bubbly manner. Though, I know from her mentioning this guy, she claims he is gay. Upon seeing his profile, I thought he looked camp.
She responded: "We should defo meet up sometime babes". Part of me was so so dissapointed, but other side I thought hes probably just very camp and they are communicating like this because of that. Rather than it been romantic. There again, there is always a chance he isn't.
Alas, I put it to rest in my head. Pushed it out of sight out of mind.
Now that her schedule is changing all the time. These such incidents are cropping back up.