Why do i look sad, while doing things consciously? I have started doing things consciously, i don't want to laugh anymore, i just want to maintain that inner peace felt in doing things meditatively.
But people have started asking that "i am looking sad".
Should i smile or should i go with the flow, i want to follow Lord Buddha.

One more thing i want to know the Essence of Lord Buddha with in few lines, i theoretically know only about focusing on breathing and vipassana meditation(which i am going to attend soon)
It is difficult to be around non-meditative people when you are meditative. Dhammapada 302 states: 


  Suffering/difficulty comes from association with unequals.


When they say: "You are looking sad", this is a reflection of their own lack of knowledge, their own sadness &amp; their own underlying fear. This has happened to me a few times before, when I used to work in an office. 

The Buddha taught that a person accomplished in meditation can in a 'finger-snap' generate metta (loving-kindness). 


  Monks, if for just the time of a finger-snap a monk produces a thought of loving kindness, develops it, gives attention to it, such a
  one is rightly called a monk. Not in vain does he meditate. He acts in
  accordance with the Master’s teaching.
  
  AN 1:6.3–5


Yes, you probably should smile, say you are fine &amp; offer an easy explanation, such as you are training yourself to work/do things with calmness &amp; without stress. 

As for the essence of the Lord Buddha, this was explained in MN 37 as: 


  Nothing is worth clinging to...