I have a BA in Pyschology, and I just TA's a class on Perception last year, in which we covered nociception. Obviously, pain in a human is different than pain in an amoeba (just as it is different in humans who lack some ability to feel pain due to a genetic condition). While you could define pain as suffering, and make suseptibility to physical pain a subset of that, I tend to view pain in terms of a reaction to stimulus that causes avoidance of injury.
Certainly, the view that pain should be viewed as a particular conscious state is problematic. That is because is is extremely difficult to operationalize a definition of a particular state, such as "being in pain." And then there is a question of states that cause a reaction without awareness versus states of which we are aware.