Does anyone know if its ok to reheat milk. when...

Does anyone know if its ok to reheat milk. when making bubs bottle i put boiled water cool that down a little add milk power and cool again for right temp. once bub has finished what ever was left i would through out. a girlfriend mentioned because i wasnt heating cold milk that i could put what was left in bottle in fridge and keep for later... is this true? does anyone else do this? Regards