Bro, you're caught already. She already know you're doing weed (and maybe worse) if she's badgering you...she just wants proof and for you to admit it.

Why do you people thinks your parents are dumb...parents know everything. They've looked at your face for 15, 20 years or however old some of you are. They know what you're supposed to look like, and what you look like when you're tired, drunk, hungry, or high. When you have kids, you'll understand.

Natural, you should just tell her..listen I smoke cannabis, nothing else. She'll stop bugging you and you'll feel better.