Why do i seem to gain weight in the evening?

Maybe water weight?: Weight gain that occurs as the day goes on (but disappears the next morning) has to be due to things in the body such as water, pee, poop, or partly disgested food. It could also be differences in morning and evening clothing, if a person is not weighed wearing the same clothes. Body water changes can be due to excess intake, diuretic use, edema problems, etc..., and should be checked by a doctor.

Fluid retention: Could be fluid retention from a relative dehydration which catches up w you by the end of the day. Cut back on caffeine and alcohol. Consider sodium restriction and drink 2-3 liters of water a day.