My DS(6) has always struggled with being able to identify emotional feelings and he's similar with regards to physical feelings. He seems to have a pretty high pain threshold so can walk over Lego without complaining, for example.
It came up in conversation a while ago that he often feels travel sick on motorways despite never having actually complained about it when we're in the car. Currently he has a big swelling where a shark tooth is intermittently poking through behind a wobbly tooth. It looks sore but he's fairly insistent it isn't...
By nature he's fairly passive, not a complainer but I'm sure at six I was much more able to express my feelings...