Can't answer as to whether you hit a nerve as I don't know, and didn't self-administer mine. But they were extremely painful when I had my loading doses, especially if I'd tense the muscle prior, Dr told me I needed to completely relax it so that could be a factor. But also, I did bruise just the once quite badly, simply because enough pressure wasn't applied afterwards, applied pressure after the others and didn't have the same issue return. Hope this helps somewhat. I was told bruising is quite normal, but again I don't self-administer so I'm unsure, also had all in my arms.
I don't know if this helps but I'v always found the upper deltoid far less painful than the thigh. If I don't use the deltoid I'v found that subQ injections with a smaller gauge insulin syringe in the belly fat to be a less uncomfortable option. I'v actually found the smaller gauge needle appropriate for IM deltoid injections as well. I think there must be less nerve damage with a smaller needle to, that would seem to make sense anyway. Being water based I don't think b12 requires a large needle like some people like to use. I'm not a medical professional so I can't advise you but just relating my personal experience with this is that 31 gauge can work just fine.
The bruising will be because you caught a blood vessel - nicked a vein so it has leaked a bit of blood into the surrounding tissue. The bruising is also likely to be the reason why the are is sore.
When I first started to SI, the first 6/7 were fine, then I did what you mentioned. When I took the needle out it did keep bleeding for a while, then turned into a bruise so you did hit a blood vessel. You were unlucky to hit it, as there aren't many in that area (according to the nurse who showed me where & how to inject)
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