Hi there,
As you have been constantly facing a weight loss and weight reduction cycle its necessary to get certain tests done.Also once you have done the tests do visit a nutritionist for an opinion.Also stress can be one reason why this might be happening.
Next Steps
Do visit a Dr. and the Dr. would guide on which tests need to be done.
Health Tips
Eat a well balanced diet,drink 12-15glasses of water and get adequate sleep 8hrs daily.