Yup, it's fermenting.  Leave it alone and you might get something interesting!   It might grow and crawl around the house!
So, chuck it.  Make a fresh (not too much) batch if you want, otherwise use wallpaper paste and some PVA mixed in.  That will keep for yonks.
DavidO

Hear hear David!

I only put the instructions for flour paste in the tutorial as there as still quite a lot of people who prefer to use this adhesive, or just want to try it.

I personally never use flour. I don't like the thought of using a "food" substance. Also as David says, it goes off much more quickly.

I have never had any problems using wallpaper paste and I always recommend it to others.

Whatever goo you decide to use, make sure you keep it in a cool place.  8)

When I started doing papier-mache about a year ago, I used flour.  I was into bas relief figures and made one of a bride and another of a groom for a wedding scene.  When I went down to the bowels of the earth (aka my basement) to check how they were drying, the groom was fine but the bride looked all gnawed and crumbly.  That's how I found out we had mice!  Why they didn't munch on the groom is a mystery; I guess the bride was just tastier.  Since then I've used powdered marble and it works just fine, though I do put on a mask when pouring it as it's supposed not to be great for breathing into the lungs.

Cheers,

LampLady (aka Elsa Rubenstein)

I've never had wallpaper paste go stinky, even after months.  I suppose it may depend on what your ambient temperature and humidity is so, yes, probably to keep it in the fridge.  Don't make too much at any one time though.  Does your paste have fungicide in it or is it a 'pure' sort which therefore could deteriorate a bit more.
David

Butterbee, it may be due to the base from which the paste is made.

I know there are at least three kinds of wallpaper paste:  vinyl-based (not recommended for PM, but I've never used it), methylcellulose-based (best for PM), and wheat-based.

Without anything to go on other than your description, I am wondering if you have the wheat-based kind?  I don't really know what I'm talking about    but if you bought the dry kind and mixed it, maybe the fungicide wasn't evenly mixed with the paste mix, or maybe it was standing long enough so the mold overwhelmed the biocide and started to take over.  When the mfgrs make the wallpaper paste, they probably expect the user to use it up reasonably soon.

Anyway, that's my guess.

Sue