Katie Foster's "over flow" is about trying to hold back overwhelming emotion and what happens when it finally gets out. The speaker compares herself to someone pressing a lid onto a boiling pan. Her hands and arms are burnt, and the pot spits and hisses, yet she keeps holding it down because she is too scared to let go in public. "Not here not yet!" captures that panic well, and the bubble in her chest and held breath make the strain feel physical.
When she reaches home or sees her mum, the pot boils over. The sobbing is loud and painful, and she admits she doesn't know why it is happening or what causes it. Instead of fighting it, she lets it run "like a river" until she goes quiet.
The mood is raw, tired and honest. By the end it softens into a simple wish for comfort: a hug from her mum, or lying flat on her bed until the humming in her head stops.