Or if you have a ten-gallon tank, run out and buy 4 gallons of drinking water. Leave the tops off in a warm place (which is easier said than done this time of the year, I know). As the water gets used, fill from you tap and treat. Let it sit open again so that free nitrogen and CO2 can escape from it and so a tiny bit of free oxygen can be absorbed by it.

She may either be sensitive to something in the water or having trouble getting enough oxygen. Water changes can help with both those conditions. Increased partial water changes (25-30-35-40-45%) which has been treated (and seasoned in an ideal world) sounds like an awfully over simplistic suggestion, but they are good things to do until something else comes to mind. :)

I'm sure you have had females that sought some solitude before dropping. That sounds like a normal and sometimes ingrained survival strategy to help more fry escape predation. A fish that is hurting from less than optimum water (and is fighting nitrate poisoning for instance) is likely to also seek shelter and less current.

I have had fish slow down too. The best I could guess is that the nitrogenous wastes were gathering in the water. At a certain osmotic level ammonia, et al, will get into a fish's blood stream. Once there those things can inhibit hemoglobin's ability to carry enough oxygen to the cells. Without that oxygen, the fish slowwwwws down. And I will confess (though embarrassed by it) to seeing evidence that there is a point of no return for the afflicted fish. That has been corroborated by others. :(

And of course maybe some fish are just more outgoing than other guppies.

So that I don't sound too judgmental about tanks needing water changes, I'm past needing to get back to doing some partial water changes today and had now better get back to them...