As Justin said, the tank should be able to handle the nitrates at 20 without showing ill effects, so take a breath and settle down! ;)

I think it is very possible to see nitrates creep from 10ish to 20ish by disturbing the rocks and sandbed as you have. You mentioned that the rearranging of the rocks = fewer dead spots now. That means you are currently blasting flow on areas of rock that have previously been detritus collectors. That would logically release some stored up nitrates. Same goes with the sand. Not sure how regularly or how seldom your sand got stirred before, but adding 20 new pounds to it could definitely cause a bit of disruption to pockets of stored muck. Just keep an eye on it, get a comparison water test from a LFS, and continue regular water changes (nothing radical needed).