You are not alone. After Covid i have been plagued with the same thing and many of my friends are too. At night i raise my bed snd use s humidifier next to me. If i wake up coughing and it wont stop draining i take Robitussin cough syrup. Something that works. I also dust my bedroom frequently especially under and around my bed. So little is known about Covid and it’s frustrating! Talk to your mom again and see an allergist. Good luck!