I'm 22 and pretty healthy but i'm always so tired! i have endometriosis. is that the reason or could it be something more serious?

May be fibromyalgia: Although endometriosis does not cause fatigue, many patients with endometriosis also have fibromyalgia. These two conditions often coexist as they may both be due to an immune system dysfunction and/or heightened neural sensitivity. You should see your medical doctor for evaluation as this is also a highly treatable condition.

Suggest evaluation: Endometriosis per se does not typically cause fatigue. It can be a mental dran, but if you are truly fatigued you should consider further evaluation, including thyroid evaluation and maybe a blood count.