After offering my Salah I always feel something is release from my body I want to find out what could be is that? And before Salah I feel so heavy and confused whether I should go for Salah to finish out my work first.
The Quran, the word of Allah says Salat (or generally any kind of remembering Allah) bring relaxation for soul of human:
الَّذِينَ آمَنُوا وَتَطْمَئِنُّ قُلُوبُهُم بِذِكْرِ اللَّـهِ ۗ أَلَا بِذِكْرِ اللَّـهِ تَطْمَئِنُّ الْقُلُوبُ
Those who have believed and whose hearts are assured by the remembrance of Allah. Unquestionably, by the remembrance of Allah hearts are assured." http://tanzil.net/#13:28
It can be due to this relaxation and assurance.