I have been undiagnosed all of my life until this month. Therefore, I have been unmedicated. I had times when I felt flat. I couldn't seem to feel happy or sad. I couldn't get excited about things I should be excited about. I just felt like a Vulcan (Star Trek), all logic with no emotion.
The reason I finally had to get help is because I went into a mixed state where I felt like I lost my mind. Followed by rapid cycling. In all of that I was having the same flat feeling as well as the up and down.
For me it has nothing to do with medicine. I can't say anything about how to make it go away only that you are not the only one.