BigSandyBalls2015 as the OP has any of this helped you?
Really if we started from a position of how could we include girls in everything, without assuming they will find out their own solutions that suit us and do not suit them, then we would make a lot faster headway.
This might involve:
Acknowledging females aged 12 to 51 (on average) will bleed once a month for a number of days. This is not a problem. It is normal and a biological reality. But it will throw up issues for the girls which mean working around any issues. If the girls were not bleeding it would most likely mean they were pregnant, underweight or ill.
Acknowledging that girls should be able to do all activities and sports at school
Acknowledging that girls may not wish to/may not be able to use tampons, this may mean girls making up time in the pool at another suitable time or doing suitable exercises to exercise their muscles/plan swimming techniques or whatever would help them to do their sport when not affected by their period
Never asking a girls parent to provide a letter as to why she cannot swim or cannot use a tampon, it should be well known by now by any PE Teacher all the reasons why this may be the case
Not assuming girls will use their periods to get out of sport (and recognizing if they do this it is themselves that they are cheating - how the hell does it affect anyone else if girls duck out of sports!)
Celebrating that we can all win if we remove any barriers to others winning (assuming we do not hurt others by doing this), that our 'can do' attitude can permeate other areas of our work and lives
I honestly do not think that this issue of very heavy periods and 'non-wellness' (really cannot think of another word for it) around menstruation is not just an issue that affects swimming. I am sure it can affects other areas of sports and also other areas of study.
The fact girls have been left to get on with it for so long is because the whole education system is set up around a 'male' model of what a student is. And this may be because for many centuries, and even in places today, educating girls was not on the agenda!