Hi, seems like you do actually need medical help. What you have described seems to be a condition called as thrombophlebitis i.e. inflammation of the vein which was used for injection as the injecting solution might be impure, irritating to internal structure of the vein etc. My concern here is if improper injection technique was used, the injectable solution might go deeper in tissues to cause further tissue damage. My sincere advice at present is enquire about thrombophobe (heparin) ointment which shall dissolve the blood clot formed at injection site & help regularise the blood flow & hence reduce the pain. You can also take simple over the counter analgesics like Ibuprofen for pain along with application of ice pack at pain site for rapid relief. You should seriously consider getting detoxed by contacting a Psychiatrist/addiction specialist. Best wishes