Germany: questions on sick leave and Reha

After only 4 weeks of sick leave, the Medizinische Dienst (MD) already called me to check on my situation and asked for the contact details of my doctors. Is it common for the MD to get involved this early in the process?

I have now passed 6 weeks of sick leave, but I haven’t received any letter or decision regarding my sickness benefit (Krankengeld).

I’m worried they might send me to a rehabilitation program (Reha) soon. If that happens, can I object if my doctor confirms that my current condition isn’t suitable for a Reha? For example, because long walking distances (e.g., to meals) could worsen my condition. Maybe I could do it later, but not right now.