Sorry you're struggling @Mkxxx but I'm with the majority here who think you are being very unreasonable. And fwiw, I'm also from a culture where families are expected to support each other, and that includes grandparents being around as a spare pair of hands for their grandkids (though also where older people are respected/ deferred to - if it's a contagious illness you have, I wouldn't be exposing an older woman to it).
You're looking after 2 kids, 4 days a week, during office hours if I've read right? And your mum is coming over in the afternoon because you're not well. Your set up is just about as easy as parenting can get.
I'm no martyr - I'd take any help that was offered to me - but if you've had kids on the understanding that you'll share them with your mum, you need to revise your understanding.
Personally, if I was her I'd miss the gym session to help you out (as long as you didn't have noro or something). But on the other hand, 8am-5pm, every Tuesday? I think she's probably earned a wee break herself?
And having read your updates/ your reply to @Longtimelurker1980 "Remember this when they are old and infirm. You reap what you sow." "Thank you for understanding. I won't hold it over her as she has helped me too."
Yikes. Your mother is effectively doing formal childcare for you (while you're on maternity leave) a full day every week. In all honesty, I don't know of any cultures where the family care practices align so well with the working day. Certainly not mine. But even from a transactional perspective, you have nothing to "hold over her". She's going well above and beyond, if she's looking after your kids, without you, a day a week.
If this is about the relationship, I suggest you change the arrangement to a more relational one - say, you hang out with your mum and your kids for a few hours, a few days a week. You pick up her shopping on the way to her house. She plays with the kids while you clean her bathroom, then she makes everyone some lunch before you help her clear up and go home to get your own house sorted and dinner started.