By nature, a child should be happy, bursting with energy and sociable. However, it can happen that a particular event affects a child. A passing, temporary feeling of upsetness should not be very alarming.
It is however important to detect any prolonged change in behaviour. In this case, if you see that your child sleeps more than usual, has a loss of appetite, is grumpy or aggressive, or if his attention span is particularly low, it would be advisable to consult a specialist to spot any disturbance before it becomes problematic.