Pekin duck help!!!!!!!! :((((

[chicksducks1363]

I have two ducklings that are both turning 4 weeks tomorrow. Just a couple days ago one of them started to limp and I have been trying to give him more vitamins (tomatoes, cottage cheese, bananas, and broccoli). But tonight he is panting really bad and his heart is beating very fast. What is wrong with him and is there a way for me to help him?!?! These two ducks are my first ducks please help me