Nitrates Off the Scale

I recently went through the exercise of rebuilding my substrate using the
Plant Guild's mix of A.R.C. Clay and P.M.D.D. A couple of days later I
retested the water and noticed that my Nitrates reading was off the scale
(44ppm using LaMotte kit). All other readings were in the normal or
acceptable range (including phosphates).

There is no evidence of any distress in the tank. Actually quite the
opposite. Fish seem healthy and plants are thriving...noticible reduction
in algae. I've done some other things recently, such as adding pH
Controller to my CO2.

My question is this: Is it possible that I'm getting some sort of false
reading? Are there other elements (such as Nitrogen) that might cause my
Nitrates to read high?

My second question is: Should I panic?