All N9XXUA are 735s...
It was most likely a 733, and you just thought it was a 735...the new livery makes it look a little shorter IMO, and the two are easy to confuse with each other as it is.
Do you remember how many rows were in the plane? As much as I'd like to see it, I'm almost positive UA
does not have a single 735 in the new livery, especially since they are dumping most of them.