Hey, how's it going? Since it starts with the lower leaves, it looks like a slight magnesium deficiency. Check the pH level in the soil. During the vegetative stage, the pH should be around 6.4-6.5, and when it goes into flowering, the pH should be between 6.8-7.0. The plant can't absorb nutrients properly if the pH is incorrect. A deficiency can occur due to an incorrect pH even if there are enough nutrients present. If the pH in the soil is around 6.5, it's a slight magnesium deficiency, and you just need to add a bit of calmag. You can usually tell there's a deficiency because the lower leaves are often affected.