Why did my pee all of a sudden start smelling? (i drink a lot of water and it doesn't hurt)

Diet. Urine odor is typically from diet, or from concentration of ammonia which is a by-product of normal metabolism that is eliminated by the kidneys as waste. If urine is more concentrated (from dehydration) this ammonia smell can be more noticeable. Otherwise, it's usually something you consumed.
Not obvious. The smell of urine is determined by its chemical composition, which is determined by several factors including diet, kidney function and hydration. If your urine smells different than usual, it could be simply changes in diet (foods like asparagus cause smell changes pretty soon after eating them) but it could also be an early sign of infection. If you have to go often or it hurts, see a doctor.