I am so very sorry
! You sound just heartbroken and Bella sounds like she was so special.
To me, this sounds like she either was stung by something, or ate a poisonous leaf/plant -- especially w/ the foaming at the mouth. The immediacy of her "quietness" with your daughter though, makes me lean more toward a sting of some kind. Sadly, she would not necessarily have to be "allergic" and have an allergic response to die from a sting. In certain bug venoms, there are neurotoxins and all sorts of nasty things, and she could have been more sensitive due to her size, due to something unknown -- and/or she could have progressed to shock simply due to some of the effects of the venom and pain.
Again, I am so sorry for your loss
!