19, female. did nothing yesterday just relaxed woke up today and feel like i did sit ups all day yesterday.

No easy answer: I know it may not seem like there is more to relay about your problem, but based on the limited description, it is impossible to say what's wrong. I suspect nothing serious and in a few hours it may feel better. If not, try a relaxing warm bath, Ibuprofen or tylenol, (acetaminophen) unless contraindicated, and good night rest and see how you feel the next A.M.

R U Sure Sit up Pain: Muscles get "sit-up sore" from use or overuse, and not from just relaxing. If you have nausea or fever consider abdominal G.I. Problems. So.. If you continue to feel pain see a doctor. When to see a doctor depends on the amount of pain. Now if a lot of pain, in a day or two if moderate to slight pain, and in a week if light pain that won't go away.