eat, eat, eat. i know i know
i lost weight by accident as well. i was already vegan for a couple years with no weight change in either direction then got a job that was a 1/2 hour walk from my house, and back, every day. silly me didn't eat more to make up for all those calories i was burning every day, and ended up going from 107 to 94 lbs. so when i figured out all my bones were sticking out more than usual and thought to weigh myself i freaked out and just started eating more. it's hard. so i ate and ate and ate and ended up with seriously bad stomach cramps
that goes away though, your body just needs to get used to more food intake, and try to make sure what you do add to your meals is healthy, don't go overboard with the sugar like i did
i'm pretty sure that's what upset my stomach, too much carbs.
so it wasn't really working as well as i'd hoped so i joined the gym and got a personal trainer for 6 sessions. woot! i gained so fast! i got up to 110 in less than 2 months. it depends on your body type of course, my trainer told me that i have the type of body that will put on muscle very quickly, so you may not gain as quickly as i did but i'm sure it will make a big difference.
not just that but i had so much more energy, i'd work out before work, come home, eat, go to work, and you'd think i'd be tired but by the time i got to work i'd be so energetic and in a really good mood.
anywho i sorta stopped going over the summer, too busy with biking stuff (bf races) and i lost half of it again lol. i'm gonna start going back as soon as biking is done.
so yeah, working out can help but you have to eat more too, especially while working out. i would get up, have breakfast, do some stuff around the house etc, then have lunch before going to the gym, then eat as soon as i got home from the gym, then have supper later on. you have to load up with carbs before working out, pasta etc, and when you get home you need lots and lots of protein. protein is essential for building muscle, without it you'll be hurting like crazy and not see much difference.