Should I be concerned about red patches after hylenex?

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. "This answer has been solicited without seeing this patient and cannot
be held as true medical advice, but only opinion. Seek in-person
treatment with a trained medical professional for appropriate care."