Hi, welcome to the forum, if tiredness is associated with mood changes then these are the symptoms of depression or anxiety disorder. Some of the causes are heredity, hormonal changes, daily stress, mental disorders, post traumatic stress disorder, obsessive compulsive disorder, phobias etc. You should find out the triggering factor and avoid it strictly.
Try relaxation therapies to avoid stress. Regular exercise will help improve circulation and make her enthusiastic. You need to share your problems with some one and put your thoughts in right way.
Generalize conditions needed to be ruled out are anemia, thyroid disorder (hypothyroidism), blood disorder and dietary deficiencies. Firstly you need to undergo routine blood tests like complete blood count, differential count, ESR, Hb% and thyroid profile etc.
I suggest her to consult a physician and get evaluated. Take care and regards.
There are so many possibilities. Hypothyroidism, adrenal fatigue, malnutrition, an undiagnosed infection. She has to see a doctor to find out what is wrong.