Dear Vishnu,
It appears that you want to activate your manipur chakra for sake of developing hunger as you mentioned that you are feeling loss of appetite.
Mantra and visualization are the tools with you can concentrate mind on any object - physical or subtle. It appears to be difficult to concentrate or meditate without using such techniques.
So far as loss of appetite, this may be because of medication that you have discussed in your question which may interfere with your digestion system. You should consult your doctor, with your complete case history, under whom treatment of your rheumatic fever and recommended injections therapy were done.