A sudden change in behavior such as being irritable, crying frequently, losing interest in play, or becoming less active is not normal if it has been persistent. It may be due to an underlying physical problem (such as anemia, poor sleep, constipation, urinary infection,
thyroid problems, chronic illness, or nutritional deficiencies), emotional stress, or a behavioral/developmental concern.
I would recommend that she be examined by a pediatrician to identify the cause. The doctor will take a detailed history, perform a physical examination, and decide if any tests are needed.