I've been told it is 'normal' when I asked another neighbour after having had the same on a few (all gone now and perfectly fine), but my mate did also re-literate the obvious - clean your needles in-between jabs, slow the injection speed, rub well after and make sure your doing it in the right place.
As I said it was the obvious - but its so easy to neglect cleaning and speed when you have a few to do.
I imagine that also having a sharp needle has alot to do with 'bruising' and swelling of tissue?
Baz