Why was the party in Chapter Six different for Nick?
One of the reasons this party is different from the others is that Tom is there. Nick notes, "Perhaps his presence gave the evening its peculiar quality of oppressiveness—it stands out in my memory from Gatsby’s other parties that summer." In general, this is similar to other parties at Gatsby's house. There are celebrities, socialites, and most behaving like drunken freeloaders. Tom's presence is the new variable.
But this party also occurs after Gatsby and Daisy have reunited. With Tom, Daisy, and Gatsby there, it might have been uncomfortable for Nick. At one point, Daisy does ask Nick to serve as somewhat of a lookout so she can be alone with Gatsby at least for a small bit of time.
If Tom had not been there, Nick would have felt more comfortable. Nick doesn't really approve of Gatsby's dealings with criminals (Wolfshiem) nor does he necessarily approve of cheating with another man's wife. But he does seem to prefer the union of Gatsby and Daisy to that of Tom and Daisy. Nick admires Gatsby's genuine romanticism. Nick does not care for Tom at all. So, it is mostly Tom's presence and the secret tension of this strange love triangle that makes Nick feel odd.
It should also be noted that these parties have served as a means for Gatsby to attain a certain social position, one that is more on Daisy's level. After he has reunited with her, the party seems like more posturing. It is all part of an ongoing act. Perhaps by this point, Nick is even more acutely aware of the superficiality of it all.