@Mkxxx
I am sorry you are not feeling well, and I hope you feel better soon. I also hope that when your husband gets home he can get stuck in so you can take yourself off for a rest or have an early night.
I am not sure it was your intention but you sounded extremely entitled in your first post. You are very, very lucky to have your mum have your kids for a full day every week FOR FREE.
They are your kids. These kinds of situations (sick kids, sick mum etc) happen to every family. It is not your mum's responsibility to change her plans and the tone in which you wrote your post was very demanding.
I am not sure about your mum's gym but at mine, missing booked classes at short notice might trigger penalties.
She might have a class that she attends that either she very much enjoys, is a social outlet for her, she meets other people there etc or she just might not want to do another day of childcare at short notice. There is nothing wrong at all with her choosing to ring fence this particular day for herself.
It is unfortunate that you are ill but many many parents have ZERO family support and have to cope with illness (either their kids or themselves). You just have a low key day, take all the meds that are allowed, and make a cheat meal for dinner. This may happen many many times over the years with young children. You can't just expect people to come in and rescue you all the time.
These are your children - strictly speaking this is a situation that you and your DH should be able to manage between yourselves.
Also saying "I would do it for my kids if they asked me to come and help". Please be careful with an attitude like this. One day - when you are feeling better - try to think about how many times your mum looked after you (and any siblings you have) when she herself was ill and she didn't have anyone rush in to help her or she had to juggle and re-arrange other commitments or pressures.
Your mum has done her time raising small kids - with you - she is allowed to prioritise herself from time to time. She is already doing you a massive favour.
An attitude of gratitude might help you.
I hope you feel better soon.