Hi April0
Theres always a come back which makes me back down such as "well I'm obviously just a massive burden", "maybe you'd be better off without me", "I don't deserve this", "it's only because I care so much, perhaps you'd prefer it if I didn't care at all"......
Afraid the above comments are some of the classics of emotional manipulation. That's why you feel guilty because that's how she wants you to feel so you will do what she wants. If this has been her way of childrearing with yourself and your brother I definitely would not be wanting her to have a child of mine for more than one day a week! Maternity leave is a very precious time with your baby but most of yours seems to have been shared with your mother!
Try to see past those posts that say things like 'we stayed at home with our children', 'children are better off with family' etc. You don't need to take on even more guilt! Times are different now and most new parents go back to work, choose good registered childcare and your baby will enjoy the activities there and will spend a day with his Nan too. Make sure you enjoy the other days with him. Equally, while I sympathise greatly with kwest, you cannot live your life putting up with a negative situation because "your mother could be dead tomorrow with no warning". You are dealing with enough guilt already.
So yes, 4 full days is saturation point especially with the extra phone calls on top. Scale back gently but firmly, try the broken record approach. Look up 'emotional manipulation' on some of the excellent psychology websites or watch some of the videos on YouTube, try looking up 'passive aggressive behaviour' too. Look at the techniques for dealing with people who show these traits and practise them. Madgran77 has suggested some great ideas and there is plenty of support here for you if you need to come back.
Wishing you all the very best, remember you are not just standing up for yourself now, but also for the sort of future you want your son to have, don't let your mum pass on the guilt to him too.