Some great advice already. I'm just going to list some things that are jumping out at me and I hope they land in the right way:
1. How long were you in the Army for? Did this mean a lot of time away from K?
2. How long have you been working two jobs?
3. How is your general mood, working such long hours?
4. How was your relationship with K before opening up to H?
5. K and H pretty much alone 'together' all day, since K is looking for work and H is a student? (i.e. IM/Skype/Phone/Chat time?)
6. Do you and H talk much? Are only K and H in an NRE love zone? How is your relationship with H? Is it what you want it to be?
I ask this because I'm wondering if there might be a pattern going on here. If you work long hours before (and maybe were away in the past, too), K might be very used to having her own time. As others have said, this can be lonely. On the other hand, maybe she loves that and gets thrown when that spontaneously changes.
I ask about your general mood because my GF's husband works long hours, like you, 3 months out of the year. He is a nightmare to live with when he's going through that. He moans and is huffy and constantly talks about his job. He sleeps all the time and basically.... doesn't encourage people to want to be around him, because he sucks the fun out of everything and brings people down. If you recognise that you might be offloading your workday stress onto K (especially if you're calling her as soon as you get out the door).... could this be changed? Hit the gym, listen to music, get home and eat first; do something to shake it off, then call for romantic, happy, light talk?
I am hearing alarm bells that K and H perhaps chatter away to each other all day and bask in NRE, whilst you have to be more and more distant because you cannot communicate all day. It concerns me, because without honest communication and ensuring that you both inject enough fun into your relationship, you could disappear into work and she could disappear into her NRE with H.
Last week I called when I got off work on Friday at 1700, I was elated that it was friday and I didn't have to work until 2200. I called K, she was asleep and I had woken her up, she was grouchy and flip.
There are probably two obvious reasons for this, outside of any depression etc. that other people have suggested.
I wouldn't put it the way BG did, but to be fair, he's always honest and this can be very helpful.
My GF is kind of like this with her husband. She loves him dearly, but she just acknowledges that the way she keeps loving him is to have a good amount of time away from him. She's happy being pretty reclusive, having quiet time to herself, doing hobbies. He's a non-stop chatterer and can be obnoxious and loud. When he arrives home early, or hangs around the house when she thought he'd be going out, she gets frustrated and snappy with him. Basically, if we're going to avoid sugar coating - she simply doesn't want to have him around a lot. Does that mean she doesn't really love him? She says she loves him deeply. Does K love you? Only she knows. Does K find your personality differences annoying at times, even if she deeply loves you? Only K knows!
The second possibility is that she might be someone who hates having her time interrupted, or her plans changed. I love being around my girlfriend. I have an amazing time with her. But, if I think I've got until 10pm alone, house to myself, me-time... and she comes home early? Yeah, sometimes I feel a bit frustrated, because I had all these plans for the last five blissful hours of solitude. Sleep, watch porn, eat ice cream and get going on a solo Netflix marathon. Certainly, I adore the ground she walks on and don't find her annoying in the slightest. For me, that issue; that grumpiness comes from my *plans* being ruined.
So, possibly lack of love OR mild irritation with your presence for personality reasons OR hates having plans/schedule changed.
Why not have a clear conversation about this - ask the questions you need to ask. Do you still love me? Does my presence annoy you? Do you just not like having your alone time spontaneously interrupted? Would it help if I text you before I call? How is it that I hurt you / you think you hurt me, etc?