While is possible that your sixline is a female and is holding eggs, there are no known labriforms (fish in the Suborder Labroidei, [i.e. wrasses, parrotfish, cichlids]) that are known to carry live young in any fashion, (There are atleast 7 different methods that fish carry "live young").
My personal guess is a swim bladder problem (probably not if it wasn't that way when you bought it) or it has eaten a lot of pods in your tank, and has filled out. Ive noticed the later with several 12-line (aka 4-line) wrasses. Both fish are closely related.