Sometimes when they learn a new skill, such as sitting up by themselves or standing, they can do it in their sleep for some time, and not work out how to get back down. Both of mine have done it, drove me nuts, it eventually died back down once they learnt to lay down again. Personally I wouldn't be going to a toddler bed that young, you'll more than likely find her asleep all over her room instead, or she could topple over the rails. I'd say she sleeps well in your bed cause as she goes through sleep cycles, she is comforted knowing you are beside her and doesn't stir as much, and naps during the day they tend to not move much at all.