In my experience: quality of the tape recorder and of the mic you use will affect how close the recording is to how you actually sound - I'm fortunate in that my dad was into this sort of thing so I have a professional quality if rather old mic and recorder - it makes a VAST difference. If this recorder is the only thing you have access to, may be worth trying to mic it up rather than use the internal mic. Expressive pieces also (IMO) suffer more from bad recording than scales or studies.
If the tape recorder you have is the only record you have of yourself, don't despair: don't compare it with what you think you sound like as you are playing (even with pretty high quality equipment, it won't sound the same) but compare day to day or over time and listen for improvement rather than the actual quality of the sound (does that make sense?)
I heard last November in close succession two different recordings of myself on camcorders: not the best recording medium at all for music! Neither of them was stunning, but the two performances were almost exactly a year apart from each other. The first actually made me cringe in places: intonation wasn't spot on, tone was so weak and wavery - ugh, did I really sound like that??! The second I was pleasantly surprised by - same type of recorder and play back as far as I'm aware, and boy was the difference HUGE or what. Still not the best quality of sound etc, but the sound I was making was so obviously SO much better than I had been making the year before - much more than I had realised.
So... get yourself a good quality tape if possible... if at all possible, buy beg steal(!) or borrow a decent mic and or tape recorder and experiment to see what positioning works best for picking up the flute... and if all else fails use it as an exercise in comparison, because things like intonation and tone will show and improvement even if the recording quality is nto that great...
HTH... sorry you've had an icky day practice-wise.

Hope your concert goes great tonight.
Sarah