One thing that will probably help is that I can hear quite a bit of noise... could be clothes and what is probably a computer fan? Get the computer at least as far away as posible... When you're recording yourself, it's tempting to have it all set up right next to each other, but try setting the computer up further away and deal with the few steps it takes to get to the mic setup.
The room could be accentuating some of the highs, but I would bet the main issue is mic placement in this case.
The 4099 is a clip-on? And you say 14th fret, but you don't say how far away... if it's clipped on and close to the strings, that would explain the metallic tone.
Try clipping it to something like a mic stand and experiment with it being a little further from the fretboard, pointing towards the body a little?
The feel free to hang a couple of blankets if you're in a small room, just to tame the high frequency reflections - obviously not as good as real treatment or a great room, but doable on a tight budget.
the room was my dressing room, small and with few reflective surfaces... but there is the noise coming from the ventilation unit from the building (can't turn it off...)
the 4099 is a clip on mic. And I put it at about 10-15 cm from the fretboard. Maybe it was too close from the fretboead, i'll try to fix that...
i've listening to my recording over and over and i think the annoying thing is that kind of "sweep" or wha effect i can hear on the guitar, like if the frequency of a filter was going up and down... but where is that coming from?
Yeah, start around a foot and see from there... You'll get more room sound further away from the instrument, but the instrument will sound more natural.
I'm not hearing the effect you mentioned, but what you describe is a comb filter sweeping that happens when there's a slight reflection off something and then as things move (the instrument or things around it), the combs change and cause that swishy sound.