Mathilde is a Yharnamites. Her husband succumbed to the beast scourge and she was widowed. Mathilde herself was inflicted with Ashen Blood and was nursed by Doctor Alfredius- her neighbour and good friend of her deceased husband.
When Alfredius used Blood Ministration to treat her disease, she started to experience nightmares, but hers were different from the one hunters usually see upon imbibing the blood, indicating something was amiss.
the Ashen Blood sickness start to manifest within her.