Well, this is my take@Candyfloss11.
You had an arrangement which has worked well for around a year - certainly from your point of view, and probably for your Mother for most of that year. You have stated that perhaps you have taken her a bit for granted so perhaps the 'assumption' she will watch your son has become more pronounced in recent months (easy to do, no-one at fault here) and perhaps it may have been not working for her as much as for you more recently. I can't say that for certain of course, but it is a possibility
What is clear to me, at least, is that things have changed because you are NOW wanting her to watch your son for fixed hours when you go back to work. That is a huge change of circumstances on your current arrangements
I'm sure she has enjoyed being able to see her Grandson and spend time getting to know him and love him and I'm also sure she has enjoyed being able to spend time with you as well, while you were on Maternity Leave. Being asked to babysit when you were 'in a pinch' was almost certainly not a problem for her, especially if she was able to say 'no' occasionally if she had other things arranged.
As someone who was a single parent with 2 kids and who is now a Grandmother to 4 boys of varying ages, I speak from experience when I say that committing to watch a grandchild while their parents work, is VERY hard work. You no longer have the flexibility to do what you want to do when you do it. You are tied to your agreement to watch the child/children if you don't want to let its parents down. It is also much more difficult to have the energy, even if you have the health still, to watch children - especially very young ones, which your son is. For many Grandparents, due to age and health it would be too much for them to do without making themselves ill. There is a reason why Mother Nature decreed that there is a cut off age after which we can't have children. So IF your Mother took this on, it would now become a great sacrifice for her - instead of the more relaxed time you have been spending with her over the past Year.
Things are now about to change drastically for you all. If I were you, I would firstly tell my Mother that I have thought about it, realised it wouldn't be fair to have her watching your son all the time you are at work and you have decided you will put him into day care.
Once you have done that - but in a SEPARATE conversion, later, you should have a heart to heart with her about what her plans are about returning home etc. Perhaps once you have actually returned to work would be the best time? If you can't afford to have her staying with you for free, if you are paying out for daycare, you could suggest that a contribution to the household expenses would be appreciated. But do not make it sound like a 'threat' or a way of getting back at her because she won't watch your son for free. On the other hand, if you CAN afford it, well maybe she can start doing some chores for you while you are out as payment in kind? Cooking the Dinner, the laundry, ironing etc? You NEED to have a heart to heart with her about the whole situation
Regardless of how much financially you are currently supporting your Mother it is not reasonable, in my opinion, to just expect her to watch your son once you return to work without payment - for the reasons I said above. It doesn't mean she doesn't love you or your Grandchild just because she asked. You are being very thoughtless, I'm sorry.
Only you know your Mother, so you are best placed as to how to have the conversations, but that's my advice.