Similar to the following:
* Arse like a blood orange
* Arse like a fresh bullet wound
* Arse like the Japanese flag

can be used to describe untold pain - often a burning sensation - when taking a dump. Spicy foods such as curry can impart such feelings and so too a bad stomach where it feels like you have taken to pissing out of your arsehole ( and the repeated wiping only serves to make it worse)

It is a reference to the visual appearance of the top of a bottle of Heinz Tomato Ketchup. More specifically a large glass and well-used bottle as found in truckers cafes where the lid is often left off and the red ketchup congeals in to something somewhat unsightly.
i.e - "Jeez, I shouldn't have had that vindaloo last night. I've got an arse like the top of a sauce bottle
by Bun001 August 1, 2012
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