What Causes Swelling And Itching On An Injection Site?

hi....im 27...M....5"5, 61 kgs... Whenever i receive injections, I get it on my buttocks. It swells a lot & remains like that for at least three days along with itching n pain.......the doctors say that this is abnormal & it only means that i'm too sensitive to even tolerate one injection......They say that the after effects should ideally be felt for a day......wot to do???????