Dead rodent or plumbing problem?

There is a rodent problem in my loft, it's been baited twice and we're just waiting for all the bait to be taken. Now yesterday i smelled an awful, rancid sort of smell in the toilet downstairs, it smells sort of like stale human feces, and the smell travelled up to the airing cupboard & to my bedroom, all of these areas are on the right side of my house where i originally heard the scuffling noises, and in the loft as well. Do you think the smell is of decomposing rats or could it actually be a problem with one of the drains or pipes? I've tried all sorts of odour eliminator/air fresheners...even bicarbonate of soda and lemon won't fix it! I don't know how to tell the difference between a dead rat smell and a sewage problem? help me!!!