Perhaps it is an implied criticism of the way you dressed the child that morning. Or maybe there's just a hint that you are too protective of him. Don't bristle with indignation, however. There's every likelihood that this sort of negative remark is prompted by her jealousy of you, and is not about anything you have done wrong.
When you think about the situation from your female employer's perspective, it's hardly surprising that she occasionally resents you. Here are some reasons why she may sometimes feel that way:
Parental guilt. The fact that your employer has taken on a nanny for her child so that she can resume her own career doesn't mean that she's entirely happy with that decision. She may feel guilty about leaving him in the care of someone else, no matter what qualifications and experience that carer has, and perhaps she feels that as the child's mother she should be the one at home looking after him. This guilt at leaving her growing child temporarily each day makes her jealous of you.
Closeness. Within a short time of taking up your post with the family, you begin to form a close emotional connection with the child - it's through that psychological connection that he starts to trust you, responds positively to you, asks you for advice when he has a problem, and looks forward to seeing you each day. This attachment enables him to thrive under your care and stimulation. But his mother may feel resentful when she sees the two of you feeling so close to each other.
Crisis reaction. There will be times when you and you employer are both in the presence of her child together, maybe during the overlap periods at the start and finish of your working day, or perhaps when you go out together on a child-centred outing. And if he faces a minor crisis, such as falling over or cutting himself or feeling anxious about something, he may first turn to you for support instead of turning to his mother. Naturally, such behaviour would make her jealous of you.
Other people. Your employer's jealousy of you can also be triggered by thoughtless comments from others. All it takes, for example, is one of the grandparents to remark that the child seems so happy and well behaved when he's with you and yet he can be so difficult when he's with his mother - she immediately starts to consider the possibility that she is less capable of raising the child than you. Anxieties about her skills as a parent can be sent into overdrive by this type of remark from other people.
Just as in children, jealousy in adults shows in a number of ways, and they won't always be obvious. For example, there can be subtle criticism of the way you do your job, even though you are sure these negative comments have no foundation. There may be a complaint made in front of the child that his room isn't tidy enough at the end of the day when you leave, or that the kitchen is not clean enough after the two of you have made biscuits together, or that you don't read enough stories to him during the day. And no matter how seriously you consider these concerns, you conclude they are totally unjustified.
Another way jealousy can show is through attempts by your employer to 'muscle in' on your plans. You might find that she stays at home longer than expected after you arrive in the morning, so that your planned programme of activities is interrupted by her insisting that she and the child do something together while you stand by and watch. This is one way the mother 'proves' to herself that he loves her more than he loves you, that deep down he prefers her company to yours. Of course she doesn't rationalise her behaviour in this way, but the truth is that jealousy may be the underlying emotion.
And then your employer might be quite open and actually tell you she feels jealous, perhaps laughing at the same time as if to make light of it. Her words could totally surprise you, because as far as you can see the mother-child relationship is strong. Yet it's all down to personal perceptions. If she worries about the quality of care she provides for her child in comparison with the high standards you set, then jealousy might set in.
Whatever way your employer's jealous shows, you have to deal with it before this emotion seriously interferes with your work (see box below). Coping with parental resentment is one of the professional skills that you should develop as part of your role as a nanny. At best, jealousy can make your job difficult, and at worst it can result in you changing to another job.