"I am in my chair."
Translation:Sono nella mia seggiola.
From a Google Images search and this dictionary (http://www.wordreference.com/iten/seggiola), I got the impression that they're more or less synonyms, but "seggiola" can be an individual bench also, and that "sedia" is much more widely used (judging from the big list of compound forms).
Possessive adjectives can be placed after the noun for emphasis, but this tends to be reserved for idiomatic expressions like "Dio mio". http://en.wikipedia.org/wiki/Italian_grammar#Possessive_adjectives