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).