Pertussis and steroids

I went to the doctor for a physical and got a DPT shot and had an allergic reaction. Needless to say the shot that was supposed to help me be healthy has made me dreadfully sick. I have pertussis (whooping cough). The doctor put me on steroids to help me get over it. I tried to talk her out of it because I heard that it will make you gain weight. We will see.