I'm a Birmingham fan who only joined up to leave a message to the poor guy earlier, but i've just read this so thought i'd leave my experience.
I met someone online when I was 20 and living in Spain, she lived local to my home in England. When I moved back, we met up one night in a club, got on well, and eventually got together. Everything was good for a few months, with the same feelings you describe (for example, you've never felt that way before and it's more than love etc).
A few months later she moved to Uni, only an hours drive away, but I didn't have a car so I had to coach it. Went with her the first time, met her new housemates etc, everything seemed good. Went up a few times and again, it seemed cool. But slowly, I was seeing photos of her, and noticing that despite us talking on Skype/Phone/Texts etc, I was starting to get really edgy when she said she was going out, especially as I experienced a lad at her uni trying it on whilst I was there!!
Anyway, eventually, we go to pick her up with her mom and dad, and I get a bit of a cold shoulder response from her. We get home, have a Chinese and the cold shoulder continues, for example i'm left downstairs whilst she goes to have a shower. I get her alone, ask her outright, she comes out with some bullshit that she's this that the other. Basically, it comes out that she doesn't want to be tied down to me whilst she's there.
That's honestly where I reckon you'll get to, honest opinion. She's at the flirting stage, i'm sure my ex was there when there were photos I saw. It was as if she was vetting them before they were being posted, and threw the odd flirty one in so that she didn't look totally innocent as I would expect a bit of flirting.
Anyway a few weeks later, she came home, think it was Christmas holidays from Uni. Saw her in a club and text her to meet me in a certain place. She came up, I held her, kissed her, and whilst she had her eyes closed, turned, and walked away, and she stood there gobsmacked. I had a text a few weeks later saying she wanted to try and see if we could get back together, I told her where to go.
The moral of this story? It's now about 7 years on, i've been married to the love of my life for a year and a half and never been happier. I realise now that what I felt, probably wasn't love and was more just 'young love'.
I thought I trusted her, totally, but the doubts I had, that you're having, are showing you that you, and I, really don't. Trust me, if you've got any worries at all, they're probably right. Not that's she's going to cheat on you, more that she'll end up messing around.
There will be someone else out there for you, and when you totally trust someone, those doubts you're getting now, will never ever appear.
Good luck with whatever you do kid, life is a big lesson that we're all learning.
