Feeling sick suddenly, throwing up clear/yellow mucus. What could be wrong with me?

You don't say exactly where the pain is. If it is in your lower right side it could be appendicitis. If it is in your upper right side, sort of under your rib cage it could be a gall bladder attack. Or it could be food poisoning, or a urinary tract infection, or a kidney stone, or ovarian cysts, or something else. You need to get it checked.