In a first 10 day vipassana meditation course I did almost continuous intensive meditation for 7 days ,I felt a strange calmness such that I could almost not feel the weight of my body , this feeling lasted till 2-3 days and then I stopped doing it, So what is that feeling called and is it common after meditation? Also after I resumed the meditation then I could not get back the same feeling (nor was I seeking for it).
The sensation of the body 'feeling heavy', very relaxed, 'numb' or almost as if it's disappearing is quite normal. When there is nothing new happening, sort of, when bodily sensations don't change, the mind 'loses interest' and kind of deactivates those sense inputs. This can be beneficial for focusing on the mind experience, since there are no longer as many body sensations impinging on the attention.
Going looking for the same sensations again can get you stuck instead of moving forward, so it sounds like a good idea not to seek it, as you mention. More likely, you may experience different types of calmness and other experiences, when you let go and are not chasing an experience you've had, previously.
The calmness you are referring to, sounds like samadhi - a deep calmness brought about by "single pointedness of mind". In deepest samadhi, absorption is so complete that all sense of "self" disappears, and subject and object are completely absorbed into each other. However, there are many kinds and levels of samadhi.
(Disclaimer: Take this answer as a -means- to explore further and -not- as a definitive answer since the range of descriptions of meditative experiences varies quite a bit from person to person.)
1. What is that feeling called?
2. Is it common after meditation?
Yes, I would say it is common and advised by the Buddha to keep developing it. (You may want to see Jhāna for more information.)
If it is accompanied by tingling limbs, tingling lips or minor headache it can be a sign of hyperventilation.
It can also be a feeling associated with detachment and depersonalization. Here is a scientific study, albeit with only 6 participants, that confirms meditation can induce depersonalization. It talks about treatment for depersonalization but personally I do not consider it a bad thing and instead part of the meditation experience.
Here is a .pdf that is more in depth: The Circumplex Structure of Depersonalization/Derealization