What Causes Swelling And Pain In Arm Post Infusion Of Drugs?

I don t know what to call this it s got me confused really.ive been a iv drug user for about 8 years now. I don t do it all the time my arms are very heath no marks or scar tissue. Never had a bad miss either until now. It didn t burn or sting no swelling not a single thing at all. It was like I didn t just do one I went to sleep woke up about 8 hours later it still wasn t hurting then after being up a couple hours it started swelling it s crazy hot now I can t bend my arm it hurts so bad it s sore up to my arm pit even. My fingers are loosing feeling as well. Please help me