Bangladesh is practically surrounded by India, so it seems strange to me that they didn't become part of India when independance was won from the British.
Bangladesh is what was formally known as "East Pakistan". Like Pakistan, when India and Pakistan became independent, they became separate countries. Bangladesh though was actually part of the same country as Pakistan, until 1971 when Bangladesh received it's independence from Pakistan. This is because they were both the Muslim majority territories in India with very large populations, while the rest of the subcontinent that became "India" has a majority Hindu population.
The old British "India" was partitioned roughly along religious and ethnic lines into India and Pakistan. Pakistan consisted of two areas, West Pakistan (today's Pakistan) and East Pakistan (today's Bangladesh).
In 1971, East Pakistan fought a war (with India on its side) for independence and became Bangladesh.
India, Pakistan, and Bangladesh were originally all the same country until 1947. In 1947 it was partitioned along religious lines with India being secular and Pakistan (including what is now Bangladesh) being muslim. Bangladesh had a much larger population, but all the power was concentrated in Pakistan due to it being the historic seat of power. Pakistan was reluctant to have universal suffrage because then the power would be in east Pakistan (Bangladesh) And not the north (present day Pakistan). This caused an increasing divide in the two regions, which eventually led to the separation, which was supported by India as a way to make Pakistan weaker as they weren't getting along terribly well at the time. So then there were 3 countries. Source India: A History, by John Keay
Can i get more elaboration on the 1971 war and the little thing of the genocide which accompanied it?
http://en.wikipedia.org/wiki/Bangladesh_Liberation_War
also why didn't india reincorporate it then?
In 1932 Round Table Conference, British regime granted Independence to Muslims/Sikhs/Christians/Parsis/Buddhists/Jains/Dalits/Adivasis in India; https://en.wikipedia.org/wiki/Communal_Award