I would suggest that seeing somebody else and the need to inform the partner are matters that should be discussed and agreed upon before entering into any firm relationship. In your case, it's not quite clear (to me) whether there was a formal dom.sub relationship or you were still in the "hold off" phase. What seems to have bothered you most was not her getting back with her ex but you not knowing or suspecting it could happen. Establishing the limits and parameters upfront would or should have eliminated that problem. Well in theory at least.