What Causes Bruise On Arm After Injecting Insulin For Type-1 Diabetes?

I ve just gotten home, taking off my jacket when I notice a strange bruise on my arm. This morning, I injected myself with insulin in the same spot the bruise is in (I m a diabetic).On the outer circle it s purple, though in the middle is a brightened pink with red dots. There s a knot where the bruise is, and I m wondering if I should consult my doctor and book an appointment? I ve never gotten a bruise from taking a shot before, and I ve been a Type One Diabetic for fifteen years.