Gosh.Maybe its a blood clot..You should get it checked right away.
Try bathin it with warm salt water as often as poss
go to the doctor or the the ER ASAP
I can't really help here but I would say if you are worried, get back to the doctor or go to the er. Were you honest with your doctor about how it happened? If not, he may not have prescribed the right antibiotic or treated you the way you needed to be treated. If it's getting bigger, I would probably get it checked out again asap. I would also stop shooting anything as it may end up leading you back to your original drug of choice. Sometimes the habit of getting the drug in our systems can be just as hard to break as the opiate itself.
Congrats on your clean time. Keep moving forward and never give up the fight. It is worth all the pain of withdrawals to live a clean life. Please stick around this forum and post often. It helps so much. Have you checked in to any NA meetings? Aftercare is almost a must in order to not relapse.
Hang in there and best of luck!
Brian