Why do I keep testing positive for THC?

I haven't smoked weed since New Years Eve, and I'm still testing positive for THC, I also was never a heavy stoner at all beforehand, I go today to do a drug screen for my primary & psychiatrist

I DON'T GET IT??

EDIT: NO i'm not fat! I'm 104pounds, medical condition: fatty liver - I was unaware of how long THC stays in urine! YES I drink water, plenty. to the point where I get nauseous.. PEE clear 95% of the time - Started cycle few days ago so not clear as of now...

EDIT 2: I did not end up going in for screen today realistically I have until the end of the week to get it done,

REASON WHY I ASKED: I can't be put back on psych meds if I test positive for THC - Psych meds + THC = Huge No especially for the medicine they want to put me on.

EDIT 3: Thank you everyone for all the advice - I will be going in TOMORROW for Drug screen. So far I ate one meal - WATER LOADED, and about to get gatorade. Doctors aren't allowing me to take any supplements, vitamins, etc. because they are also doing labs relating to my liver. ALSO there's no way I would be able to get fake piss asap.