Is my girlfriend exhibiting signs of PTSD or is it just depression?!

Hey, my girlfriend used to be in the British Army (Royal Artillery, Light Gunner) from when she was 17 until she was honourably discharged one year ago at 22. I met her in college where we study an Access course to get into uni, but to be honest, she's actually gone really quiet lately. She used to be really disciplined and top of her work, in fact, she was doing the best in the class and has some great uni offers, but she's not been doing great in the past month and letting it get ahead of her. She's just really moody, but she's really obsessed with order, keeping her bedroom in order and she has this urge to go out for a run before she sleeps. She has had nightmares a few times, but not too often. And the other day there was a police helicopter looking for some guy who was on the run and she seemed to be "in a different world" for about 15 minutes, before she just acted like as if nothing happened.

She doesn't say much about her Army career, when we first started the course, she was all "yeah I loved my time there" with a great smile but that's it really it. I know she wasn't on the actual frontlines, but she did serve in Iraq and Afghanistan and you'd think she'd have a lot to say, but in the English lessons, she never writes about it or anything. I'm just wondering if my girlfriend might have PTSD and how I can go about helping her?

I'm 24 and I've never been in the military, so I don't know how things like this work. I probably don't understand.