My 14 Year Old Daughter Has Episodes Of Severe Exhaustion/

My 14 year old daughter has episodes of severe exhaustion/ sleepiness where it's nearly impossible to wake her, her eyes are e,extremely red, she slurs and staggers. It truly appears she's on something. She has been sent home from school a couple of times and I had to take her for drug testing - which was negative. She has depression and takes surpassing a n d trazidone because she has trouble falling asleep. What could this be and what can we do so she can function?