My hair has been getting progressively drier over the last few months, and I can't figure out why! I don't wash it every day, I condition when I do wash, I take omega-3 pills every day (for other reasons, but for a while they made my hair really shiny!), and now, my hair is constantly dry.
    I tried a hot oil treatment, but it didn't really work; my hair was just greasy for a day and then back to dry. Any help here?

Your "dry" hair might be hair that is coated with product.  If your conditioner has protein in it, too much protein can cause brittle hair.  

Try using a clarifying shampoo or a vinegar rinse and see if that helps.