Then you're most likely fine. Like what I've said on my previous reply, it's most likely just them asking for proof that you're not selling some illegal stuff, or they're going to try to tax you. Either, or both. In this case, since they're already questioning you anyway, it might be the best move to just be completely honest with your answer. You probably could get more into trouble if you try to make some excuse just so you wouldn't pay income taxes.