Hylenex is a brand of hyaluronidase. Any time an injection is done in the skin you risk bleeding, which will result in bruising, and swelling. If this doesn't resolve within a few days, or if it worsens or starts to sting or have a burning sensation, I would suggest returning to your doctor. Otherwise, I would assume it's just bruising from the bleeding and swelling which will resolve in the coming days.
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