Poor little thing, great you are taking her to the vet, hopefully they will be able to diagnose what is going on.
Being so young it is a worry, I wouldn't force her if she is in so much pain that she is crying, best to feed little portions often.
I hope you can get into your vet asap, in the mean time, you should soak the kibble in water to a mushy consistency as this will help with easier digestion, you may also want to look at putting her on a grain free kibble. Perhaps grains are causing her issue?
Keep her food as bland as possible.
Also, it is not a good idea to swap brands all the time, they need time to get used to one and if you decide it is not the right one, then you need to phase in another one slowly.