24m, 17 days after circumcision. over a week ago a longish horizontal 'hump' appeared under the remaining foreskin on the top of shaft but can't pull back that far to check it. it's not painful at all. should i be worried or will it go away?

This soon after a surgery, if you have concerns about the outcome, you should talk with your surgeon. My impression from your description is that it probably is not anything to be concerned about, but if it persists or worsens or does not resolve, then you should show it to your surgeon. Good luck.

Hard to tell without exam, Use warm soaks for a few days and see If not better in a week pl see your surgeon.