Away on travels - is she cheating? (is it normal to be paranoid?)

We've been dating for a year.
I've been away from her, travelling for a few months. One more to go, and then I'm back home to see her once again.

We seem very compatible and I think I may be in love for the first time.

Over the past month, however, I've been getting a sinking feeling in my gut. There's something wrong. Is it my insecurity? Is it paranoia? Or am I onto something here?

Here are the 'signs', as I see them:

1. She has a history of promiscuity. In itself, this isn't a bad thing. In fact, it can be good: she has enough experience to know what she wants.
However, when added to everything else, I can't ignore it.

2. She's insecure and has low self-esteem.

3. Since the beginning of our relationship, she has sought reassurance from me that I wasn't cheating - and never would. I thought her paranoia was endearing, but could it be a projection of her guilt? (I realise how hypocritical this sounds since I am essentially in her shoes now.)

4. She has admitted to cheating on her previous long-term boyfriend. She didn't define it as 'cheating', but her (unconvincing) rationalisations don't change facts.

5. Last time I was away for several months, she admitted to seeing a guy. She told me about it of her own accord. Nothing reportedly happened. She just saw a movie with him, and then saw him again once. She apparently told him she had a boyfriend and that she'd never do anything to hurt me.
At the time, I didn't think anything of it. I was either stupid, naive, or both. Truth is, I've never been the jealous/paranoid type. Until now. So back then, I took her story at face value and didn't give it a second thought.
Now, I realise that I may have passed up an opportunity to not only question her about it, but display some boundaries. At the very least, I could have showed her I cared enough to be ever-so slightly jealous. I did none of these things, and as a result, I may have reinforced her belief that doing this type of stuff is okay (it isn't - it's a step towards infidelity at best), or pushed her away with my aloofness.

6. She's from overseas and came here with no local friends. She has felt bad about not having her own social circle. This has made her depressed in the past. Could this make her more receptive to the advances of other men, under the guise of 'friendship'? Gut feeling says yes. There is precedent for it after all.

7. She has bought lingerie "for herself" while I'm away travelling. Is she simply indulging herself - which is perfectly fine, to be encouraged even - or is she doing it for someone else?
She has shown me pictures of herself in it, but... alarm bells are still ringing.

8. No news from her during (or after) New Year's eve. New Years is a crap time to be away from your partner: drinking, partying - plenty of opportunity for mistakes to be made.

9. She has been working excessively while I'm away. (Hospitality industry.) She has told me that she feels so exhausted as a result that her libido is gone. At the same time, she has been going out with coworkers. I'm glad that her social life is taking off, but I hear things about the workplace being a breeding ground for infidelity...

I'm fully aware of how insecure I sound, but I just can't shake the feeling that something is awry. And if other people's experiences are anything to go by, I would be an idiot to ignore my gut feelings. At the same time, I feel guilt for not giving my gf the benefit of the doubt.

I never thought of myself as the type to have trust/jealousy issues, but I guess I've never been sufficiently invested in a relationship to feel paranoid before.
Well, as far as new experiences go, this sucks.

So... is it normal for there to be potential 'signs' of infidelity like this, or is there something out of the ordinary going on?