What are the causes and effects of too low hemoglobin?

Low hemoglobin. Anemia may be caused by blood loss perhaps through GI bleeding or decreased blood production in bone marrow. Fatigue and shortness of breath are most common dymptoms.
Varied... Causes are many (either production or destruction problem): iron deficiency (diet related), inherited conditions (sickle cell), auto-immune (hemolysis). Symptoms can include: easy fatigue and loss of energy, rapid heart beat (particularly with exercise), shortness of breath, headache, difficulty concentrating, dizziness, pale skin, leg cramps, and insomnia.