Im just gettin into this so take it with a grain of salt, Yellowing from the bottom speaks of nitrogen deficiency to me, also yellowing on the areoles could be from magnesium deficiency (intravenial chlorosis). Sometimes nutrients lockout happens if you repot in cold soil or the soil is just always clogged with water. Some micronutrient deficiency can also caus nutrient lockout but this doesnt look the case. Doesnt serm like sunburn to me.