My son had a runny nose today and was grumpy for the most part. Guess what...he wasn't that hungry! This boy LOVES to eat, so that is how I know when he is sick, because his appetite is less than usual. My two year old son is no exception from other kids...the appetite is the first to go and last to return.
Kids and babies with special needs especially tend to be less hungry when they are sick or when they aren't feeling well. "Not feeling well" could be a temporary state such as being upset, having indegestion, breathing fast, being tired, and the list goes on. I think for babies with sensory processing problems, especially those with medical needs or prematurity, it doesn't take much to make them feel "not well". It could be something as simple as gas, reflux, or over-stimulation. The following link includes signs of over-stimulation that a baby may display. This is important to know, because until a baby is calmer, he probably will not have a good sucking pattern, especially if respiratory problems exist.
Alertness & Crying
Also included in this link are the arousal levels of a baby. Studies show that babies are more coordinated with their sucking patterns when in a quiet alert state or even a bit drowsy, but not as well in the other arousal states (deep sleep, light sleep, active alert).