What causes tender aching skin and muscles especially on lower left abdomen? Other symptoms include blood, undigested food and lots of mucus in stool

Your Doctor can help. Blood in the stool is always worry some. You can go and see your gp and ask him to examine you and check you blood for any problems. For a young person, you are already affected by more problems than i would expect. You need to exercise daily and eat healthy diet(this is just general advice which may get your mind off the illnesses that are affecting you a bit sooner than later).