1 . Most simply answer , Yes could just be your body telling you need a break . 2 . Did you change your workout time ? Mid to late evening workouts , even early evening for some people will charge you up for hours afterwards . 3 . Stress ? Anything happen to stress you out this week at work or home life ? 4 . Maybe see your doctor and get a blood panel to check your normals Blood glucose , liver function , immunity , iron etc etc . It ' s really good to get this done a couple of times a year even if you feel great ! 5 . Diet . Haven ' t started smashing a ton of sugar late in the evening have you ? I ' ve run into all these problems before , as a runner and a lifter , and I think most active people will agree . <| sep |> You think it ' s for you but it ' s for them . What better way to corner advertising than to tell your competition in bulk what to do and how to do it .