If this is a conventional lava rock shower filter, a nitrite spike is expected even in a mature system. That is because the pathway for the lava rock shower filter to get rid of nitrAtes is to first convert the nitrAte back to nitrIte, then from nitrIte back to nitrogen gas.
Of course it could also be a test interference since there are a variety of things that test as nitrite, notably most amines. But my bet is on the nitrAte conversion process going through nitrITe to get rid of nitrAte.
Your koiphen chemist and environmental scientist.