Well, its good u have taken admission , CSE from NITs is a good option . As far as , getting into IITs is concerned , there are seats vacant coz all the quota seats have nt been filled up , as many students cud nt clear the cut off. 1100 of the quota seats were nt filled up, even when the cut off fr quota ws lowered.
Some of the obc seats have been transferred to general , and rest are fr the preparatory courses. At your rank , i think it would be quite low probability getting into IITs even in the second counselling. At present , be content with ur current situation as its good too, and hope fr the best.
Best of Luck!!