Stranger Danger!
Alright, so we’re going to be a tad bit judgmental here. Let us get this straight: you, a new mother, decided to leave your one-month-old child with your MIL because you wanted to celebrate your husband’s birthday? Why could you guys just celebrate it at home? Why did you have to leave at all?
It’s no wonder your child was screaming their head off by the time you got back, presumably several hours after leaving them with grandma. She probably did a good job babysitting, but in the eyes of your newborn, she was a complete stranger! Good job traumatizing your kid before they could even roll over!