i'm sure you are worrying, not daft. this is a very stressful situation to be in. why would your son say you did it if it was an accident - or do you feel the school and social services are overreacting? do you live with your son, have access (if you are seperated/divorced) and were around at the time said bump appeared? where is your son now? what is he like? what is his mother like/your relationship like? all of these things have a bearing on what is happening to you and would help us give you advice.
as for the legal stuff...you will attend the station tomorrow, be interviewed again, maybe and/or charged with an offence (maybe assault, maybe something else - you will find out soon enough). do not admit to anything at this stage and be very careful about what you are saying to people - to family involved/under caution/in the station/to anybody professional involved. it will be best to discuss things with your solicitor at length prior to further discussion about what has happened and the likely outcomes, but by all means state you don't know how the bump got there/were not involved/do not know why your son would say such a thing. your behaviour and conversations with people will be scrutinised very closely at present and should conduct yourself accordingly. if you are innocent and there is a simple explanation, this will be sorted out in due course. if you have a complicated life/background/relationship and have been a bit of an arse, it's going to be far more difficult for you and complicated. again, a bit more detail is required to advise you.