You've punctuated a valve/vein and its caused some internal bleeding. That said if your arm becomes more painful at the injection site, it becomes redder or inflammed then seek medical assistances. If you fear judgement then look up for drug and alcohol counselling services in your state. They often provide medical services for users. PM if your in Sydney and i can suggest some places if you need help.
There are several anti-bruising creams you can use. Eleuphrat cream is incredible however if your not unemployed then it'll cost a fortune to get a prescription filled. A lo-tech but incredible powerful alternative is some aloe vera. Just apply it directly to your skin. (as long as you don't have any open cuts that are glistening/not closed up).
Lastly you might have missed, injecting some of the solution outside of the vein thus causing the lump of various bodily fluids and such. It should have drained away after a while.