お誕生日おめでとう河川ちゃん

Kasen doesn't remember much about her seventh birthday - just that kaasan left to go get vegetables for the stew they were going to eat and, when she returned, she wasn't kaasan anymore. 

She doesn't know what she did wrong,  but Kasen's sure it was particularly bad because, until well past the new year, her mother couldn't even look at her. 

Kasen stopped calling her 母さん that day when she became 母上 instead.