Probably you failed to pay bill in time, so that switch (or whatever the technical term is) they usually use to limit the UL bandwith got reset somehow, which wasn't the case until now. Or probably they realised after six months that they forgot to limit your speed. Its BSNL, you can't have a clear cut answer, but 99% chance its because of their dumbness or lazyness.
There is nothing accurate about BSNL. It is all based on luck.
Each month at least 3 - 4 times my FUP gets reset for a few hours and I get 2 Mbps. Then they send me email, SMS saying that I have crossed the FUP and if I want to pay for higher speed. Luckily they block my internet only once a month.