The live rock you just added may have some decaying matter. Hence this explains why there could be some Ammonia & Nitrite in the system.
If your tank has been well cycled, it should be able to convert these ammonia/nitrites very quickly. If not, continue to cycle the tank.
How long have you been cycling your tank previously? Did you have any biological media or rocks in the system previously? Are you dosing any bacteria product to boost the beneficial bacteria population?