In Act 4, Professor Higgins and Colonel Pickering delight in their successes. The have effectively transformed Eliza from a cockney flower girl, to a beautiful lady. As they toast their success and delight in what they've done, they forget to take into account any of the work Eliza has put into the experiment. They do not notice that she is visibly upset and crying "what is to become of me?"
She knows that things will not be the same for her. In her new state she cannot return to life as a flower girl, but she does not have the means to continue life in this manner. As Higgins offers that her solution is to "marry well" she becomes more distraught and upset with him
"I sold flowers. I didn't sell myself. Now you've made a lady of me I'm not fit to sell anything else."
She returns all of the jewels he gave her and leaves Higgins' house.