Whether they came to you bloated looking or not, the fact that you feed each of them a big flake (not once but twice a day) is saying a lot to me about why that one is still bloated looking. Flakes are not a good idea for single bettas and are even worse for groups of them because there is no way of monitoring which one gets how much. In a group there is always going to be one that gets more than the others it is just how it is nothing you can really do to stop it. Which means that you will end up with a very bloated betta after a while. Their stomach is only around the size of their eyeball and anything over that is too much. Flakes swell much more than most pellets which means that even though that flake may not look like it is too big, it is. The good brands of betta pellets take all this into account and they do not swell like that once in the betta's stomach which keeps the betta from bloating. You may not have really noticed a problem with your bettas before now which is why you continue to support using flakes instead of switching to pellets. Your previous bettas probably were lucky enough to not get really bloated but I would guarantee they were a little "heavier" than what they should have been normally.
WIthout pictures it is impossible to tell whether the bloating with your female is food related or something worse. In a situation like you have them in, it is possible for it to be an early sign of internal injuries and fluid retention from fighting.
My suggestion for you is to fast the tank for at least 3 days to see if the one female starts to look thinner. I also advise you to purchase some small pellets so your bettas have a better diet and to avoid this problem in the future. I also advise you (like I did in your thread about adding platys) to get a couple more females if you want this tank to work long term.
