Hi Leeloo, the reason most people don't see axolotl poo is because it disintegrates pretty fast depending on what they eat. If they eat meat it tends to be a stringy version of whatever meat strip they've been given, worm casings have appeared in our tank and sometimes we find a rugby shaped pink/brownish coloured poo which does disintegrate when you try and pick it up with a turkey baster (can be a bit frustrating sometimes
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12 days isn't long. The constant rolling over upside down means whatever's bothering her hasn't been sorted out.
Putting her back in the tank will not fix the problem.
I've had my axies in the the fridge for up to 3.5 weeks at any one time, some people have had theirs in the fridge for the whole summer due to temperature problems. Two of our axolotls I've kept separated from their tank for up to eight weeks in lowered water, not in the fridge but the temperatures were lower than 15C, until I was completely sure they had no problems (ie sideway swimming, fungus and were eating properly).
I suggest leaving her in the fridge for at least a week more, lowered water rather than high, and see if she can last longer than 12-24 hours upside down, and keep us posted. The longer she can stay upright the better it is for her (and not just 24hours).
Moving her back to the tank which may be in the middle of cycling, while the temperature is warmer, may cause her more stress than good.
Sometimes all you need when things like this happen are time and patience.