This is totally normal, D.. 2 1/2 is a great age to begin potty training but only a handful get it right away. Most will show initial enthusiasm, then back slide. It doesn't mean you're doing it wrong. It just means she was all excited about the concept... then discovered it was inconvenient. Plus once the initial excitement wears off (which naturally gets them focused) it is genuinely hard for them to remember. Like any game they play (which is how this seems to them).
You can gently but firmly let her know that underwear is not for peeing in -- it's only for when she ready to do the pee in the potty. Then put her back in pull-ups. Tell her to let you know when she wants to try again. In a few weeks you might ask her if she'd like to try again.
Meanwhile, give her books, point out other kids using the potty. It takes a while for them to get the concept that this is the NEW normal way of doing things. After all, they've been going in a diaper since birth. Next, it takes a while to develop the habit of mindfulness to get to the potty BEFORE they have an accident. Usually they are totally absorbed in whatever play they are doing so at first you will have to send her in every so often to go. But if she won't do it, she's not ready. Just keep a cheerful neutrality about it, to avoid the power struggles.
I teach a two-three year old parent participation preschool class, and believe me most of the kids are still in pull-ups, in various stages of toilet learning. So your girl is not falling behind. For more info about potty training, I have some here:
http://www.earthskids.com/devspecissues.aspx