What are most common causes for a dull pain in lower back? Fam hx of back issues (father). Discomfort eases when I change position, exp this 3 days

Lifting or strain. AS a young woman the most common cause of problems is overuse. This could be in form of exercise or of work etc. The historical nature of your parent with back pain is not usually related. Do not sleep on your abdomen and if this persist see your doc.