Numbness when shooting coke usually indicates hitting a nerve, like the median, ulnar, or radial nerve. It could be a missed or partially missed shot though. If it continues swelling, if your arm starts changing color or getting pale, you'll probably want to get that looked at. I would not redose at this time and give priority to watching what happens there first. Always put safety first.
If you're going to inject you might want to invest in a copy of Gray's Anatomy or a similar book, filled with images of where the veins, arteries, nerves, muscles, etc are located in your body.
Here's a discussion of the same question from 2011:
(backstory, prolly TLDC) ... Although I've been around other IV drug users for a long time, I'm new to doing it myself. I've been IV'ing cocaine occasionally. It's a once in a while kind of thing, and I'm always cautious about the amounts I do and about the common sense guidelines of IV drug use. Besides the occasional miss, because of unsteady hands (witch I have even when sober, unfortunately), things usually go pretty smoothly.
(the important part) Tonight, however, I used a vein on the underside of my right wrist, near the hand (I have very few visible veins, and I was giving...