Seriously FUCKING rant

Okay so I left my 4 month old with my mom while I took a short nap. 
I heard him crying. Didn't think anything of it. I just thought she was letting him fuss a little bit 
I ignored it for 20+ minutes until I got annoyed at the fact that she wasn't soothing him. 
What do I walk into? A BABY BY HIMSELF IN A HOUSE BY. HIM. SELF. 
(Technically not really but fucking seriously?) I search all over the house for ANYONE including my mother. No one to be found. I call her and she said she "forgot" about him 
I am so fucking pissed off its not even funny 
They just.. Left him.... 
And she said "people make mistakes" NOT LIKE THAT!!!! wtf