Another mum I know offered to help at one school event last year because they were short staffed. Since then she gets asked first for everything. Trips, cake sales, WhatsApp admin, last minute cover. She said it feels less like helping out and more like she has accidentally taken on a second unpaid job.
It made me realise the same thing is happening to me. I helped once because I could, and now it is assumed I always can. I do work, I do have other commitments, but apparently being organised means being available.
I am happy to help sometimes, but I do not want to be the default forever while others never get asked.