Feeling unsupported

This post is a bit long and emiotional (thanks pregnancy hormones) so i apologise in advance. This pregnancy has been horrible from go to woe...horrible morning sickness to horrible eskemia pain from my fibroids and have been hospitalised multiple times and had to be given some type of morphine drug to help with the constant pain im in. My husband and i had a serious chat with my mother about taking on the responsibility of looking after my 2 year old son because i am struggling. She agreed and had re arranged her schedule which i was so greatful. I did say that i will care for my son as much as i can because i want to still be useful as best as i can. Everything was going fine until now....she has started making all these plans with other people which means when i ask if shed be able to watch my son for 3 hrs in the morning she will say no sorry i cant, im doing such and such... (shes retired, its not like its work...just social stuff) She assured me in the beginning her main concern was was that i feel supported...but i can tell shes getting tired of it even though its only for a few hours here and there a couple days a week... She is still aware that im still in alot of pain, im not keeping it a secret. I just feel so disappointed that her knowing my situation, doesnt feel the need to WANT to help me out. Im sure alot of you will suggest that i talk to her but she's not good with any sort of criticism, even in the gentlest of forms. I really dont want to disrupt the peace right before our new baby arrives...Also my husband works full time and on his days off he does absolutely everything. He's also a little annoyed and said he will talk to her about it but i said no because i just dont want her to get defensive and upset... What should i do? Im thinking ill just take what i can get and lower my expectationsto prevent disappointment.