Thank you for writing in.
between 40 and 50 while sleeping is perfectly normal.
Feeling lightheaded and sudden rise in heart rate to 120 with drop in blood pressure suggests one of the two things in your daughter's case: postural hypotension
due to salt depletion and second unlikely but possible problem can be with autonomic nervous system called autonomic neuropathy. You have not written how much she weighed before she lost 70 pounds and her height. Nutrition specially micronutrients deficiency is quite common after such surgery. You ought to show her to her doctor and take advice about diet she has to take now. In adolescents of this age worrying about small things is not common and it is called adolescent adjustment reaction. Treat her as a friend and equal and not child anymore, this problem will get sorted out.
With Best Wishes.
Dr Anil Grover