I would stop worrying about who reported you. It's bound to make you feel paranoid. It could be anyone whow is concerned about your child, including your mother or your partner (or the child's father, if that's not your current partner), a neighbour or a friend, even a client who has seen toys or pictures around the house. They may have the best intention of protecting your child rather than harming you. It's very well to say you only work when your son is away from home, but I have had punters turn up out of the blue when drunk, horny or just plain selfish, and there is not much one can do about it.
Important is how you deal with the problem. Being honest is best, but there is a way of phrasing "honest" so it doesn't bite you in the bum. I would get in touch with Citizen's Advice, either in person, by phone or via email, and ask where you stand and how you should respond. I know it's a massive pain in the arse, but somebody thinks they are looking out for a vulnerable child, and that's better than the whole neighbourhood turning a blind eye while kids are neglected, as is so often the case.
Just find out how to deal with it, and things should be ok.