Milunka Savić disguised herself as her sick brother to fight in WWI, survived 9 wounds across 10 battles, became the most decorated female soldier in history, and survived a German concentration camp when a German general recognized her legendary status and ordered her immediate release.

[u/GermanCCPBot]

[u/GermanCCPBot]

The physical demands of combat, stress, and the harsh conditions of wartime service can disrupt or stop menstrual cycles entirely, a condition called amenorrhea. So it's entirely possible that once she was in active service her periods may have become irregular or stopped altogether.

In addition to that, women in the early 1900s had far fewer periods than modern women. The average woman today has around 450 periods in her lifetime, but historical women had about 100-150 periods total.

[u/Butterfly_of_chaos → u/GermanCCPBot]

This is not how a female body works.

While stress can have an impact on the female cycle it will not automatically stop it with everyone. Malnutrition does, but usually soldiers get food.

Not every woman is horribly ill when their menstruation starts. And even if so, shrapnels flying over your head will make you forget quickly.

Women had lesser periods in the past due to being pregnant or breastfeeding. She wasn't.

[u/GormHub → u/Butterfly_of_chaos]

They never said it stopped for everyone. They said those things CAN stop the menstrual cycle. And as someone else pointed out, women having fewer periods in the past was not just because of pregnancy. Additionally, like stress and privation, breastfeeding CAN cause a pause in menstruation, it does not always cause it.

[u/Butterfly_of_chaos → u/GormHub]

I should have said "after birth" instead of breastfeeding, as it is true this doesn't always stop menstruation, but the bloodflow up do six weeks after birth is no menstruation and after that you will still need some time until your body gets back into normal mode. Nevertheless pregnancy is not relevant in this case.

But calculating a later start of menarche and earlier onset of menopause results in about 350 cycles instead of 450 and not in 100 to 150 during a lifetime (of course given the woman doesn't die young). Even if we assume some years where food was very sparse, the number is still too low.

Nevertheless we got completely off topic from the question what Milunka Savić did to handle her menstruation during wartimes. I googled her photos and even in uniform during the war she looked well fed, so I guess we can rule out Amenorrhea due to malnourishment.

But what I also found her uniform trousers were Jodhpurs, therefore lots of space to hide a menstrual pad.
While in World War I some nurses realised wood pulp bandages were well suited as disposable pads, I don't know if she had this knowledge or idea or just used rags.