I am hoping to find answers to this question soon after my son is tested and we get to talk to a neuropsychologist/professor who I would assume would very knowledgeable about this.
I am not sure about this, but I always thought it was SPD if it was causing problems in every day life. I have always been highly sensitive/overexcitable in some ways and it didn't cause me to have any great difficulty, at least nothing that I couldn't work around. But my son pointed out something to me that really made me think about this.
We were getting ready to go to his first private swimming lesson (45 minute drive and the lessons are $20 for a half-hour) and I realized that we would have just enough time to get there if I didn't have trouble finding the place, so I started getting nervous. I already had a headache and was moving a little slower than usual because of it. The television was on and my son asked if I heard what was just said on it and I said no because I was so focused on getting ready and I had a headache. He said, "Good, now go try to play that song (on the piano) I have been working on." The song was a challenging one and he had not played it well at all one day, but played it perfectly the next at his piano lesson. I had tried to make him continue practicing even though he told me he was tired and distracted. I knew that I would have had trouble with this because I would not have been able to tune out the headache pain and the worry about getting some place on time. He had found a way to really make me understand how his sensory issues affect his ability to do things consistently.
I think the difference is that in his case, he deals with more pain than I do. In addition to the migraines, he gets muscle pain any time he does more than the usual amount of walking. Yesterday we went to the zoo and then a museum. He really enjoyed the museum but had to sit down to rest his legs because they hurt. His dad and I don't have this problem but I am sure if I did it would be similar to how I deal with headache pain. I think pain affects the way I take in sensory information. I have always known that it is best to not do things like paying bills when I have a migraine because I am more likely to make a mistake. I don't like to drive when I am in pain because I feel it takes my attention away from driving sometimes and I think even my reaction time might be a little different. Pain slows me down.