My cat has diarrhea, has lost a lot of weight very fast, is very lethargic and wants to eat or chew on grass.?

My cat is 7 years old & has been acting very strange lately. He has lost a significant amount of weight in a short period of time, abnormally tired, has terrible diarrhea and anytime the door is opens he shoots out the door so he can eat/chew on grass. He didn't used to do this but l have noticed a huge change in every part of his personality in the pasdt few months. He was taken to the vet like 2 or 3 months ago and they said he didn't have any parasites or anything and that he was in good health and gave him an antibiotic shot. It helped for a few weeks than started again. Can someone please give me some tips if they have experienced this with their cat PLEASE tell me what I should do?