What Causes Rashes All Over The Body After Taking Penicillin Shot?

Hello i am an 16 year old male i have just recovred from strep throat due to a penicillin shoot. A few short hours later i developed a rash almost all over my body and it feels like something is poking me all over the doctors said it came from taking to hot of a shower. I want to know if thats the only possible way they came and how can i get rid of this nightmare?