This might be useless because I was on Citalopram (for depression) not Fluoxetine
But -
When I started on them (it was either late October or early November, it's slightly hazy in my head) I felt absolutely awful, really hopeless and miserable
They started kicking in after a couple of weeks, and within about 4-6 weeks (December) I felt happy. Not just on an even keel happy, but really over the top happy. At the time, I thought 'wow this drug is a miracle'. Looking back, I think artificial is a good word to describe it. It certainly wan't real happiness. It was an artificial high, and I was very much right at the beginning of getting better
But then that high wore off. The real miserable, hopeless feelings didn't come back, but I started feeling flat because the high wasn't there any more. By the time I had been on them 3-4 months, I felt much better than I had in October. But again, looking back i definitely wasn't better at all. I was getting there but very slowly. I was still dog tired (sometimes sleeping 14-15 hours straight), little appetite, the flat feeling and disconnected
I don't think I was 'better' (back to my old self) until about April/May time, when I actually often felt genuinely happy and comfortable, on an even keel.
I absolutely think that the anti-depressant worked and it was largely responsible for getting me better, but it didn't work in a straightforward 'get a little better week by week' way, it created basically this but with an extra dip between the peak and the cafe 
Anyway, that was a long winded way of saying - maybe you're experiencing someting like i did? An initial happy feeling which wears off after a while?