What to do with a friend..?

She probably don't want to seem like she is all n your nuts when you have a girlfriend. She probably was beginning to like you but pulled herself away because you stated that you only want to have a platonic friendship. It could be that she doesn't want to annoy you too much. There could be a number of reason. I would say don't worry because you have your girl. When you see her just say hi! and keep it moving, dude.