need advice, please help me out..

so i was having a conversation with my friend about weed and my mom herd the entire thing, im pretty sure she knows, but i kind of played it off saying that hes a stoner and im not. shes just waiting for me to mess up, and she said that she is going to drug test me tomorrow, what should i do? i dont know if i should just tell her. or what, because she is totally against it..please offer some insight ents..