You should have put your age and past medical history.
Essentially the reason why the left ovary preserved was to give you an advantage of natural estrogen
from the left out ovary. Unless that you may have menopausal symptoms called 'Surgical Menopause'. Also they do not seem to have any reason to remove the left when it was not diseased.
The current changes in the left out may be because of absence of other ovary and is still producing follicles. Though this is complex with no free fluid and no para ovarian fluid the explanation appears benign. Definitely the left ovary is not large but with it fixed to the neighboring tissues, it needs a close follow up before they decide to operate and remove it. The point that there is weight loss
and prolonged diarrhoea
Let the doctor discuss with you the benefits vs risks and then you can take the final decision.