I haven't had issues with losing weight, so I can't really comment on that (but I'm sure another lady on here can).
You can definitely be petite on top, but not bottom, or vice versa. Petite isn't only about height, it's about proportion. A lot of women believe petite means "short and thin," and this just isn't the case. My friend Margaret is 5'10" and we've found that petite sized tops fit her best a lot of times. "Regular" sized tops are often too long for her from bust to shoulder, so it looks as if someone just needs to pull the shirt up higher to get the armholes to fit under the arms right and around the chest, but then there's all this extra room in the shoulders. All her height is in her legs. She also wears a misses size 12-14 on top, so again, petite is not about size. If I were you, I'd go to a store that has the same item available in both petite and regular (Nordstrom, Ann Taylor and Talbot's usually do), and try both of them on for comparison. This might help you decide if petite really is a better fit for you, because you would have a direct comparison of the same item. Also, if you have a friend that has a good eye for fit, you could take them along for opinions. Does this make sense?
It's too bad Shiny just went on vacation, she's the resident "petite" sizing expert around here. :)