My kitten is sick,please help!?

I recently found my cat of 9 years dead and that cat was my world. My sister got me a kitten and I am so protective over him because of what happened. Well we got him his first shot and he was fine but when we got his second one he had an allergic reaction and couldn't breathe so when we got him his last shot they gave him steroids and benadril to help prevent it. Its been like 3 weeks since then and I noticed a huge bump on his back,so big that I can see it when he is walking. My stupid sister has known about it and didn't say anything and she said all she knows is that he can die if its a cyst because you cant amputate his back. Our vet is closed right now so I cant call and Im only 15 so I cant take him to an emergency place and he is only a kitten still. What should I do or do you know any information??Please help me I cant loose my kitten...