6 days ago I gave myself a 1ml B12 injection in my deltoid, I now have a tensed strip of muscle in the deltoid, no pain or discomfort but it is tense?

Bolus. You may have inadvertently injected either into a fat pad or you injected too much fluid too quickly into the muscle. It is also possible that you missed the muscle and injected into the dermal layer of your skin. In most cases the will absorb into local tissues over the next 2-4 weeks.