^ Only reliable way to be sure it is not an artery is to check for no pulse in the vessel, and you can get a good idea for what veins feel like after a while, kinda bouncy, even non-visible ones - the other signs are indicators but by no means 100%.
Your feet veins are fragile; even if you registered, you could have nicked the vein or the pressure of the injection could have caused at least a martial miss. It could also be a histamine release, which sounds very feasible given the itching and bumpy red rash, or even an allergic reaction to a cut. Feet are generally pretty sensitive too so it could just be a general reaction to the shot. How is it now?
Keep an eye - signs of infection are redress/swelling/heat/pain, so get it checked out if you are concerned or if the rash doesn't fade in a day or so.
Injecting in your feet is really dodgy; fragile veins, easy to miss, more likely to get infected and not heal due to poor blood supply and increased chance of developing a blood clot (deep vein thrombosis or DVT). I would seriously consider your IV use; believe me, been there and know it is easier said than done - but you don't want to move onto even more dangerous places. I know it is not that simple but I really think you are heading into seriously damagin yourself if you continue. If you must shoot there, be meticulous in making your shot as sterile as possible and always ALWAYS use a new needle, even if you have a shot in it and you've missed a few times. Micron filter, alcohol swab and leave to dry for 30 seconds, sterile water etc.. have you looked at a vein map for other potential sites?
Have you tried plugging?
Related reading:
IV megathread
Reemember, we can give advice on possible causes, but we're not a medical forum and even a doctor couldn't diagnose a rash over the net especially without pictures! Good luck
