Respectfully, what is your training around disinfection and sterilization protocols? What special equipment are you using to filter the air in the room you work in? What tools do you have at your disposal to clean and sterilize your space, tools, linens, etc?
Having antibodies to the virus that causes COVID-19 may provide protection from getting infected with the virus again. But even if it does, we do not know how much protection the antibodies may provide or how long this protection may last. There's simply not enough science, and you're cherry-picking what you're choosing to believe.
Wearing a mask only at the client's request puts both of you in danger. And your grandmother and other innocent people you later come into contact with are also put into danger.
I understand this is an income stream and that must be hard, but if you have to keep working please be safer.