A couple of suggestions to add to the others.
First - I endorse the "keep nagging" option. Take the child by the hand, lead her to the dirty undies and stand there, making her do it right. Then walk the dirty laundry into the laundry room and put them where they are supposed to be. It is not YOUR job to scrub out stains from everyone else's laundry. How else are they going to learn how to do their own laundry when they are adults?
Second - I would buy "Depends" full undies, if she can't do this. They are actually very effective. Or failing that, packets of disposable undies for period times. If periods are really heavy, she needs to talk to you about it because that can be a big problem. I remember going through some rough times where I was literally wearing multiple pads, or pads plus tampons and still having accidents. With hindsight, I should have seen a doctor. Only my parents didn't take me to doctors unless it was really serious. Come to think of it, my kidney problems were identified because I took myself to the doctors who ordered the tests that found it. I still went through the surgery on my parents' insurance (it was before we had our current public health system). But I digress - try Depends.
THird - teach her some fast techniques for cleaning the undies quickly and easily, then equip your laundry with general supplies for making the job easier in other clothing areas too.
Fast technique No 1 - when cleaning blood especially out of clothing (especially cotton or poly-cotton) use SOAP.
Step 1 - rinse out the undies under the tap. If you hold the stain (especially if it's fresh) right against the stream under the tap, it can wash it out almost completely. But sometimes it's a bit stubborn. That's when you grab the cake of soap and rub it on the stain directly, like you're using the soap like it's a crayon and colouring in the undies. Then finger and thumb - rub to lather up the soap. Then rinse.
That should be enough, even if the stain is not completely gone. There is still enough soap there as residue, for the washing machine to have a bit of extra help.
Fast technique no 2 - wet the undies down and rinse. Then leave them to soak in enzyme soak or oxy-action.
Fast technique no 3 - spray the stains with vinegar, then leave the clothing to soak.
overall rule - ALWAYS USE COLD WATER! And when you do your laundry, cold-wash. At the most, tepid. Hot wash is only for grease removal in clothing where it's already been through a cold wash and got everything else out. Warm water cooks protein, and the human body puts out protein in a lot of its wastes. Cook it, and you're stuck with the stain (or the smell). Have you ever had a t-shirt which you wash thoroughly, then you put on and as it warms up on your body, it immediately begins to smell like bad BO? It's because the sweat got cooked in at an earlier stage. Vinegar can still help even if it gets this bad.
The vinegar spray (or splash) - we keep a bottle of No Frills vinegar in the laundry. We have some in a spray bottle for ease of use. It's this vinegar we use for the Ham Bag (they should be rinsed in a mix of vinegar and water before being put over the ham). Use the vinegar spray on any bodily fluid whatsoever. I include sweat in this.
To treat clothing - you spray with vinegar, then you can just dump it in the laundry tub (or washing machine) and leave it until washing day. Doesn't matter if it dries.
If it's really bad - then spray with vinegar and use enzyme soak.
I was once given a suit coat that was badly contaminated with cigarette smoke and perspiration. The coat was really revolting, and I was told to get it cleaned if I could, or throw it away (the expectation was that it would be thrown away).
I soaked the whole coat in vinegar. Then I put it into a tepid bath with enzyme soak. A lot of it. After 24 hours, it went into the washing machine for a full COLD wash. You should have seen the colour of the water! YUK!
The coat came out crumpled and still a bit smelly, so I repeated the process - vinegar soak, then enzyme soak, then another wash. This time the coat came out with no odour at all - no cigarette smoke smell, no perspiration smell. And a stronger shade of black - all the brown had come away in the wash.
So if the method worked for that coat, it will work for anything.
Marg