Help [Semi-Urgent] Is this a medical emergency or should I sleep it off?

Unfortunately I made A very poor choice and IV'd (or so i thought) medication. I think I may of injected 4mg of Dilaudid into an artery, there is a huge bump on my forearm in an irregular shape. It looks like a massive mosquito bight I would say mabye 4cm across. There is also lots of little bumps all around it too. I thought I was in the vein when I injected but when I was dose I got a very weird pricing type pain throughout my arm. I have no pain now and feel fine but this lump is scaring me. When I push down on the forearm it turns white and it turns back to pink within 10 seconds. Im not sure if this is just placebo but im feeling a great pressure in the back on my neck now. Help please D: