What could cause constent fatigue?

Many: Chronic fatigue has a broad range of diagnosis ranging from anemia to thyroid disorders. Would recommend that you see your primary to get some initial workup including blood tests.

See PCP: Because fatigue can happen from any organ not functioning properly, start the evaluation by meeting with your pcp. Your pcp will take you history and do a physical, which will help direct further evaluations that will lead to the diagnosis. If you start your evaluation at a specialist, it is possible you won't get your answer.

Multifactorial: Chronic fatigue syndrome is a multifactorial consequence of hormonal, immune system and nervous system dysfunction resulting from genetic predispositions and dietary and environmental insults. These factors impair the patient's ability to produce energy and they end up chronically fatigued. Poor diet, suboptimal sleep, food and other allergies contribute.