How does a doctors diagnosis work?

I've an appointment tomorrow and I'm worried if I say something along the lines of 'I felt pretty low last night', the doc will brand me with severe depression. How does it work? I'm going in tomorrow because I'm worried I might have something seriously wrong with me physically. As stated in a previous question, inside my head feels congested; like it needs to be cleaned. It's like it needs something to wash the sludge away to restore clarity. How does it work?