What kind of pain are you having - sharp, dull, crampy? Does it come and go or is it constant? Any problems going to the bathroom? Change in BMs?
I had about 2 feet of colon removed, but I had plenty to lose. After a lifetime of constipation, it was severely redundant (stretched out). It took a good 2 months before things finally calmed down pain-wise. It was mostly crampy pain. The colon is one organ that really doesn't like being messed with, and tends to go on strike for a while after surgery. It's possible it just hasn't gotten its act together yet. The good thing is that your pain levels are going in the right direction: down.
I have no idea why you would have a narrow colon, but it's likely just the way you're built. Did your surgeon happen to mention if you had a lot of scar tissue in the diseased area? That would definitely narrow things down. Seven inches really isn't that large of a resection piece. Some people have had the majority of their colons removed and the small intestine hooked directly up to the rectum. They may end up with chronic diarrhea, but they're still alive. The human body really is remarkable in that it keeps on going after all kinds of insults and organ removals.
Give it another week or so, and if things aren't still improving it's reasonable to schedule another followup with your doctor. You might ask him for a copy of the operative report and have him review it with you. Because I've had so many surgeries, I always keep a copy of the operative and pathology reports for my own records. Sometimes it's surprising how much they differ from what the surgeon actually tells you.