Toward Ecosophy

If it was something self-generated the nails in the bedside jars would have been affected, since you're presumably spending many hours each night in close proximity to them. The same is true of most kinds of attacks, since the hours of darkness are the most common period for etheric attacks to manifest -- the sun tends to break up dense etheric forms, making attack difficult in daylight. Thus the rusting of the nails in the amulet is probably a function of some kind of etheric squalor in an area you visit, and the change in the effect means that you're successfully clearing it up.