Any medicine including Henna/mehendi can cause skin allergies. Even infection in any part of your body can cause allergy
.In your case the particular batch of Henna / mehendi can be the culprit.
Why dont you go in for healthy lifestlye rather than taking medication.
Its not good to rapidly loose weight.
Duramine it self have several side effects. Its upto you and your treating physician to weigh the pro's and cons of the medications . Please consult him or her again and follow a life style modification plan.