Sinking Feeling
Imagine you are in the comfort of your own home and just went for a massive dirty shit. On completion and full ruination of the bowl you look around to realise there is no toilet roll. After the initial panic you decide that you must leave the toilet and hunt for toilet roll. On standing up you feel your arse cheeks sandwich together at which point you have that sinking feeling.
Better check first for toilet roll, you don't want to get that sinking feeling.