Silly question, but are you allowing the test strips to develope completely before you take a reading? Like everything else in this world, those strips need a little time to develope and aren't "instant read" like Litmus Paper (though that would be nice).

I can't explain the sudden skyrocket any other way, that and everything in your tank "should" be dead with 160ppm of Nitrate. I'd test again and see what's what to be sure.

A more regular water change regiment would be advised, though I know most of us at one time or another have missed a water change for various reasons.