I see a doctor.
The doctor is checking my.....
Sick? Condition? Body? Cause of sick? What is the best word for describing the doctor is checking my problem.
I would like the doctor become the first subject in the sentence.
I see a doctor.
This is a bit unusual in present tense. It would either mean that you literally see a doctor standing in front of you OR that you habitually visit a doctor "I see a doctor regularly for my allergies."
The other place we find the present tense is in the imperative.
See a doctor if your rash does not go away.
This would be someone telling you to visit a doctor if you do not get better on your own.
I saw/will see a doctor.
This means that you have a problem and visited/intend to visit a doctor.
The doctor is checking my...
You would probably only use the present tense if you were having a conversation with someone at the same time the doctor was checking your _____. Past or future are more likely.
The doctor checked my vital signs. (vital signs = temperature, pulse, blood pressure, etc.)
The doctor checked my condition. (condition = overall health)
The doctor examined me/my body/my injured arm. (examined = the act of looking at your body)
"The doctor will check/examine" would work as well, to describe an upcoming visit."
If it is a routine checkup, and you really wanted to stick with your "The doctor is checking my..." construct for some reason, you could say:
The doctor is checking my health.
If there's something in particular you are seeing the doctor about (say, a pain in your left side, or a high fever), and you wanted a multi-purpose word to use, you could say either:
The doctor is checking my condition.
or, in the case of an undiagnosed illness:
The doctor is checking my symptoms.
Otherwise, I would be specific:
The doctor is checking my left knee.
NOTE: I've written these in the present tense, to match your question. I'm imagining someone saying these things when the appointment occurs in the near future. In context, the sentences might be said in a way like this:
Ned: I'm leaving work early today because I have a doctor's appointment.
Ted: Oh, what's wrong?
Ned: Nothing too serious. The doctor is checking my...