Why has my 3 year old started wetting herself again??

My 3 year old daughter has been dry day and night for the past 16 months and she did it really quick with no hle what so ever which I thought was fantastic......... But........ Over the past couple of weeks she's been wetting herself for absolutely no reason and not telling us until we notice. We really try not no scald her as I know that this is not the way to go about it. plus this only happens when she's around us (She has nursery and private chilldminder mon - fri). It's like she just forgets. please help