Thanks for using Healthcaremagic,
Your symptoms are suggest possibly you are having vertigo
which is related to posture(Benign positional Vertigo).
You should consult a Physician and ENt surgeon for proper evaluation and treatment.
is just borderline low that may not be cause for your losing balance, if required proper medicines may be given to correct it.
Please do not start any treatment without consulting Physician.
If you are underweight
you need to take proper high calories and high protein diet along with exercise regularly to gain weight.
Hope this helps
Dr.Akhilesh Dubey M.D