Hello ma am and welcome. Heparin cayses the thinning of blood, and as heparin was given at a particular site that region can have slight bleeding due to two factors: 1. There is ann artificial wound caused by the injection, 2. The blood has thinned from the effect of heparin. Thus resulting in the hematoma. I do not think it is dangerous, but it needs to be adequately drained. Hope this helps. Best wishes.