Kyouseiteki ni Akuyaku Reijou ni Sareteita node Mazu wa Okayu wo Tabeyou to Omoimasu.

After being reincarnated in a game world, Ravie Heathfen, has been forced to play the role of the villainous noble lady after the game's heroine placed a slave curse on her. When the magic suddenly came undone, she gained back her bodily freedom and also memories of her past life but she has to make sure that the heroine doesn't find out or she'll lose her head! She wants to escape immediately but it's just impossible with the horribly skinny state her body is currently in. So for now, she'll just have to settle with eating porridge first and start building back her physical strength! But her fiancé the prince, the muscular knight, and others would unexpectedly come and start quarrelling with her. It seems that our skinny young lady's escape plan will be filled with plenty of great difficulties! Source: MU