radiation and chemo has a tendency to cause low blood pressure particularly if you are not drinking 2-3 quarts of liquids each day. 48 oz seems low, can you increase your intake of water? The high calorie drink should help. Good luck.
Regards JoAnne

JoAnne - Caregiver to husband, cancer rt. tonsil, mets to soft palate, BOT, 7 lymph nodes - T3N2BM0, stage 4. Robotic assisted surgery, radical neck dissection 2/06; 30 IMTX treatments and 4 cycles of cisplatin completed June 06.