I too bruise - I was told "some people just bruise easily".
Overall, I found changing needles every time helped.
When I first started injecting, the advice was to pinch and inject. The advice to inch seems to have gone out of the window as needles have got shorter. However, I found I was more likely to bruise if I did not pinch. This may be because I am slim but could be worth trying.
Otherwise, it is not something to be concerned about - you have just injected into a blood vessel.