why am I so exhausted?

I am new to fitnessblender, so I purchased "FB Low Impact". In the beginning I felt amazing and high energy, but suddenly I felt exhausted and couldn't do simple things that I could before.

I thoght "oh, maybe I need some extra rest" so, that what did. Now I start again and feel weaker again.

I have 2 rest days and never push myself to much.

What I do wrong?

thank you

(sorry for my english, I am English speaker )