I just started working out, why do i feel tired and very worn out afterwards?this is mainly if i work out longer than twenty minutes at a time.get better

Multiple causes: There are many reasons for fatigue with exercise. If one has just started working out , the intensity of work out may simply be too high and needs to be reduced. Slowly increasing the intensity from a lower level. Medical conditions that can also lead to excess fatigue. A medical evaluation prior to further participation would be warranted especially if decreasing doesn't improve or if older.