Injecting - what's wrong with swim's arm?

[banana pancake]

swim has been trying to figure out what this means for a long time now. sometimes swim shoots up heroin and her arm becomes very itchy. it also burns a lot. it starts to swell up a lot. a blotchy pattern appears on her arm too. swim thinks it's very uncomfortable. today her chest and face got red when this happened. her heart beats quickly when this happens too. people have told swim it means she missed, but swim still feels high when this happens to her arm. what exactly does this mean?