^ Hmm that is strange, especially if you have been shooting for that long with no problems, but there are a few things that could have happened..
Sometimes you can have a partial miss, where you register but the needle is only half in, or it partially passes through the opposite wall of the vein, so some of your shot leaks into the tissue next to your vein. This can cause pain and sometimes leave a lump, depending how much of your shot was missed. Treat it like a standard missed shot - warm compress, watch for signs of infection (redness, swelling, heat, pain) and get it checked out if you are at all concerned.
Sometimes the needle can be bumping up against a valve, which can cause pain too. Sometimes if a vein is becoming thrombosed or scarred then you can get shooting pains as you try to force liquid into it, but if that was the case there would have been resistance..
The other possibility is that you hit an artery. Your brachial artery runs along the medial asoect of your arm at your antecubital fossa (crook of elbow) but it is usually pretty deep and I would be surprised if you had hit it by mistake when you are used to hitting that vein..
Injecting into an artery is painful as the arterial walls have their own nerve supply. The colour of blood when you register is not a reliable indicator at all, but arteries will be pulsatile so best bet for anyone shooting is to always make absolutely sure beforehand. It doesn't sound like you hit an artery though, but it is a cause of a painful shot. so I thought I should mention it for anyone else reading. If you do hit one you need to immediately stop and remove the needle, apply firm pressure and get it checked out at the ER - arteries can spasm, cutting off the blood supply and you can potentially lose a limb.
Switching sites sounds like a plan, it's good rotate anyway and maybe the vein had become slightly damaged or inflammed (phlebitis) and needs to recover. Does the vein still feel absolutely normal?
Make sure you always use a fresh needle too, even if you miss - they become blunt after just one or two times and can damage your veins.
Scary needle pic:
I hope everything is okay!