I'd bet you are giving too much greens. Ease off a bit. Give less until things firm up.

I'm not sure if spinach is the best for a guinea pig. I'd give clean and dry lettuce, a carrot chunk, or a bit of red/green pepper. We've trained our pig (or she's trained us!) to give a small "green" feeding in the early morning and late afternoon.

  My piggies had the same issue with celery & iceburg lettuce, so now those are out of the diet.  Spinach is good for them though, lots of vit. C!  I agree that you should stop any "treats" for a day of so & see if that clears it up.  If it does, you can re-introduce it slowly.