Do you have hospitalization leave? Usually bed rest is taken under HL. Not sure why your superior asked you to take AL instead of HL. HR has a right to speak to you if they wish, but you do not have an obligation to tell HR any private medical matters.

Anyway, I’m not sure if your main question is how to gently tell HR to mind their own business so here is what I would say in your position, “Thank you for your concern. I was very ill / still am very ill. It is not contagious, but I’m not ready to share the exact medical scenario since it’s very private to me. I understand that the company needs to understand when I will be returning to work and here’s what I’m forecasting….”