When you add LR to an established tank you are supposed to cure it first. Even if it is advertised as "cured" you should cure it yourself just to be certain. What it sounds like happened was there was some die-off from the rock and it put your tank through a mini-cycle.
Next time put the LR in a seperate tank or container of saltwater (at your tank's SG) for a week running with a powerhead and skimmer, if possible. This should allow it to cure enough to add it to your tank.