What black flying insect can land and bite without you feeling it?

Several times a year, my mom gets bit by a mysterious insect. Once bitten, she has extreme pain and swelling for 5 to 14 days. The insect leaves fang pinhole marks. It always bites her on a major pulse point on the body. Usually on the carotid artery. She lives in KY on a property that is partially cleared and has dense woods. Any help would be greatly appreciated. The bites are serious enough to send her to the ER.