Doublemint my story is horribly similar, except it was me 
PIL lived not far outside Oxford and as they were expecting us at suppertime we had lunch in Oxford itself and bought a few bits and pieces in the Covered Market.
Very taken with the huge fresh raspberries, plums and figs, I had a lovely feast in the car, feeling very smug about all the vitamins.
Met PIL-to-be, liked them, all went well. Everyone went to bed at a reasonable hour.
As soon as I lay down to sleep, I felt the most ominous shifting in my guts, and realised with horror I was going to have to use the bathroom.
Nothing so excruciating as trying to pass esssentially a cow pat volume of hot, soft poop - with accompanying bovine wind - in the utterly silent, middle-of-nowhere, immaculate bathroom next to PILs bedroom. Giving birth to two children with only gas and air was far less painful.
On flushing - oh, cringe - the toilet bowl filled to the point where I thought it would overflow. And didn't go away. So I'm using the toilet brush as a plunger, in agonies of shame at realising PILs must be able to hear everything through the adjoining wall.
Eventually the blockage clears and thankfully I find bleach in the cupboard and ensure everything is restored to civilisation, leaving the window open a crack to deal with the poisonous stench.
Coming downstairs to breakfast in the morning, FIL claps DH on the back, saying "Still producing the biggest turds known to mankind, I see!" and to me, "How ever do you put up with him, we heard him banging around clearing up the evidence in the night!"