How to get my mother to stop lying to me?

My mother and father are divorced and they each have one child with them, my sister with my dad and me with my mom. They pay child support to each other and I asked my mother for 20$ for valentines. She said she didn't have 20$ and she couldn''t afford to give it to me. Now I've seen the child support checks and I know she could give me the money. My dad gets 40$ a week from her and she gets 140$ a week from him! She get's food stamps and works ful-time and overtime so I know she has enough money to pay the 568$ rent. She makes 325.75$ a week and still tells me she can't afford to give me 20$, now normally I'd be fine with this but when I said she gets child support every week she said she has to give him some to. I said that I'm positive you get alo more than him and she lied right to my face and said "No I don't there about equal." Now I know this isn't big lie but I clean around the house, cook, I do EVERYTHING and she just sits around ordering me and smokes all day! I fricken can't deal with it anymore, please help!