Dear M.,
It sounds like your mother and grandmother are judgmental about you working and leaving your son with them, even though they are extremely supportive at the same time. They are doing an amazing job with their support.
I went through a lot of wierd dynamics with my mom when my children were born. there are a few good books out there about mother-daughter relationships that you can check out on Amazon. anyway,there is no real way to straighten them out except by saying the whole truth about how you feel (you can also put it in writing). To which, they will probably tell you not to feel that way. Also think about your own feelings of guilt for not being around. Because whenever you point a finger, three are pointing right back at you. So it is really important to figure out what is going on with yourself.
Recognize that they are limited in this respect, and that you love them. They are loving your child and caring for him when a lot of grandmothers and great grandmothers would not be doing that job. They love you and don't mean to get a rise out of you. They may have some envy about how you turned out. From understanding what is really going on compassion can grow from that.
There can be so much garbage interwoven in loving relationships, so it is important to acknowledge it and try to weed out what is possible but let it go when you can't get to it.
I think when you stop feeling guilty, you will no longer care what they say because what they are saying are just statements of facts. As to the question of whether "mummy and daddy are mistreating you?" that simply has to stop, and it is not a joke. While they are entitled to "spoil" him and it is your role to discipline him, it is not right for them to set up your son to take your discipline as mistreatment. They have to stop saying that, and I would correct them each time by explaining how they are misdirecting your son. To this they will probably say that you take things too seriously. But as mother, it is your right and they will recognize it (even if it is subconscious).
I hope my take on the matter was helpful. Good luck with everything.
Ana M