When i abused heroin 3years ago, right after the iv injection, my veins on my right bicep went totally red. it was thick red lines. what happend?

This is simple,: As the substance and possible the impurities within induced and irritation / injury of the inner lining of the vein. When this happens blood will clot in the vein and you get an inflammation too called superficial phlebitis: red and painful. Usually a self limiting condition helped with motrin/aleve and local compression. If infection sets "suppurative phlebitis" u need antibiotics too!