How do I fence on my period without worrying about leaking through white fencing pants?
You can fence completely normally on your period; it just takes a little planning, and plenty of fencers do it every month at practice and at national events. The worries are real: white knickers (fencing pants), deep lunges, two or three hours of sweat, and not always an easy chance to change. The approach that holds up best is layering: a main form of protection plus a backup layer, so a small leak never reaches your whites.
Tampons and menstrual cups move with you and do not shift during lunges. If you have never used one, try it on a non-fencing day first, and follow the package directions on how long to wear it. Period underwear, especially sporty or boy-short cuts, works alone on light days or as the backup layer on heavy ones. If you prefer pads, choose ones with wings and wear snug compression or cycling shorts over them to hold them in place; overnight pads give more coverage. Dark shorts under your knickers also hide a small leak. On heavy days, a common combination is tampon or cup, period underwear and dark shorts.
Keep a small kit in your fencing bag: spare supplies, wipes, a change of underwear and shorts, and a few small disposal bags for bathrooms without a bin. If the club bathroom has no bin, tell the coach, or ask a parent or older teammate to mention it; it is a basic facilities fix and any decent club will sort it immediately. At tournaments there is lots of waiting between pools and elimination bouts, so go between bouts rather than when you are about to be called, and watch the bout order. If you are caught short, tell your coach or the referee before your bout starts instead of disappearing.
If a leak does happen, it happens to athletes in every sport, and nobody at a club will think twice. Rinse blood out with cold water as soon as you can, and keep a spare pair of knickers or borrow club ones. Cramps can be handled however you normally manage them; pain that regularly stops you from training or normal life is worth a visit to a doctor. Parents: keep the bag stocked quietly and keep it matter-of-fact. One separate note: a chest protector is its own piece of kit, and women and girls are required to wear one in competition, so add it to the gear list.