K.M.
Needing to suck may indicate a cranial fault- very very common slight mialignment of the cranial bones.One of the adjustments to fix this actually uses thumb sucking as part of the adjustment, as that effects the sphenoid basilar junction.
Sucking on fingers or thumb puts digestive enzymes on those tissues, and can have consequences, as they are not suppose to be there.
If she calms instantly why withhold this? Really?
Please let her have it, until you can get her to a Webster, SOT Cranipath Chiropractor. The only ones trained to fix this, in the mean time sucking on a pacifer helps her.
All this crying is her only way of communicating to you. Listen, please.
best, k