It depends on how your MC perceives "femininity". Different women perceive it differently. But I'm trying to think of a woman who links her menstruation to femininity and I'm struggling. Even women I know who have had hysterectomies haven't really mourned the loss of their menstruation, and are as comfortable with their femininity as before. So I dunno that there's a link between the two.
The only thing I can think of that would equate femininity and periods would be sort of round about if you live in a society where your only value as a human is in being able to provide offspring for a man. Value = Fertility = Proper Menstrual Cycle. If you lose your periods, you know you can't reproduce at the moment, and then, you have no value to your society. This might equate to a loss of femininity if you believe femininity is the way one advertises fertility. Fortunately, as a society, we've moved way past that (and I hope that holds with the current administration trying to return us to medieval times). Given the woman is a soldier, it sounds like that's not how the OP's society works, either.
For myself, I thought of my period as a sign of womanhood the very first time it happened. As in, "Ooohhhh I am now A WOMAN, DUN DUN DUUUNNN!" Subsequent periods quickly lost their sheen (period-shits, 'nuff said).
When I had to tell my parents so I could get the essentials on the shopping list, I was given the whole "you're a woman, now!" etc. speech. Mothers I know, now, don't do that kind of thing anymore. They're telling their daughters early and there's been no awe about it. One girl actually got the explanation and the various options, then said, "Seriously? That's what's going to happen? You're saying we can send people to the moon, but we haven't figure out a better way to deal with
this?!"
Sounds to me like it hasn't really registered with her, which seems natural given all the other stresses in her life. I'm assuming she's not sexually active, given that the possibility of pregnancy hasn't occurred to her. In those circumstances, the main concern might be that there's something medically wrong. Depending on her personality, that might worry her or she might brush it off. Her reaction to her mother's inquiry depends on their relationship and her age; as a teenager, I'd have been excruciatingly embarrassed by such a question from my mother, but as an adult I would have been less bothered.
In my twenties, my mother would have been told nothing--not when it stopped and not when it started back up. That's between me, my doctor, and my partner (strictly on a need-to-know basis). I, personally, would have been worried about a medical problem if it suddenly stopped. Given there's a military doctor about who could tell me there was nothing wrong with me but stress, I would stop worrying about the period and start focusing on doing my job and staying alive. Also, I'd be thrilled about no longer needing to deal with my period in a war zone. If only my period would just stop anytime I was too stressed out to deal with it... Let's all work on that as our next evolutionary leap!
IF I were desperately trying to get pregnant, I'd be sad that I'm not ovulating... not that my period is gone. The period isn't the THING; it's a side effect of fertility. You don't get upset when inconvenient side effects go missing. You get upset about systems not working as they're supposed to.
Also, feeling beautiful and radiant is a huge component of the feminine energy - speaking as someone with a day job in the beauty industry. During a period, all sorts of hormones and neutrotransmitters cyclically crash and a lot of women feel really nasty because of it. I'm not suggesting this as an absolute, but it's harder to feel prettier when you're breaking out, bloated, and feel like staying under the blankets all day.
If femininity is associated with being "pretty" and dainty and delicate and lacy and dolled up, etc., in this world, then just being in a constant war zone, wearing military gear, going without makeup or complicated hairstyles, being dirty all the time, not having my my nails done... would make me feel much less feminine than losing my period would.
If I'm older, then, out of the war and in a relationship with someone I know wants to start a family (and I also want to start a family), then I might be concerned about still not having ovulated, and I'd worry a bit about how to tell him that I wasn't sure I could get pregnant. If I started ovulating again, I'd probably feel relieved.
Is it
necessary that this woman feel like she's lost her femininity? I don't see why losing your period or being a soldier, for that matter (even though I gave the example) should make you feel that you've lost your femininity. Femininity is a state of mind, and every woman's version of feminine is different.
Personally speaking, unless I'm eleven and expecting one or trying to get pregnant, I don't want to read about periods. Without knowing how greatly this whole plot point affects your story, I'd say just leave out the talk about periods. Everyone reads for different reasons, but for me, reading is about escapism, and periods are the opposite of escapism. Like, a line maybe about how she wasn't even having periods anymore (The End) would be okay, but as woman, if it went on very long, I'd be all, "OMG, why is she thinking about her period all the time! Pay attention to the war!"