So, it is just misunderstanding, just like what other mama felt. Try to say something good to comfort your parents in law, try to think this incident from another angle... it is not easy to take care of little baby as they are getting old; they are not your maid but they are really to do such good thing without asking any reward.
I am almost same age with you (should be older), and I fully understand that it is quite tiring to take care of kids as we are not young and not that physically fit. But try to enjoy the family time with your kid as they grow fast. Your baby is lucky as s/he has so many people are kind with him/her.