My 15 year old girlfriend is taking warfarin for a blood clot and has burning face, dehydration, shivering, loss of appetite. what does this mean ?

Shivering and: loss of appetite in a 15 year old girl can be due to a kidney infection especially if preceded by burning on urination or associated with flank / back pain. The burning in the face may be a manifestation of a fever (check temperature). It would be appropriate to see a doctor or at least discuss these symptoms with the doctor who placed her on the coumadin (warfarin).