Small tank, all live rock, good possibility. When you start adding stuff, test for a week or two after adding anything. If you see a spike then go from there. Being as it was so quick you might have a soft population of bacteria so any additions will have to be spaced apart pretty well and you should monitor the water.
I've been testing my live rock I bought used from a few weeks ago every single day, I watched the ammonia spike a little and then no nitrites but alot of nitrates. Then moved the rock into my tank and its all 0's across the board. Live rock, depending on how well its transported can pretty much insta cycle your tank.