Two examples of septicemia, one mostly on the fins, one on the body. I wish I kept the pics of the bloody spotting on my boy's head, he's all better now! As you can see, it is a very blood-red color, and both of the fish look very poor off. After a month with continued bleeding and no stoppage, I reckon she'd have gone into dropsy or simply passed on by now. I think you might be the mom of a very pretty purple-ish marble girl
It looks like you got her young. Bettas sometimes undergo dramatic color changes as they mature. As I said, I've seen some really profound changes, and many foods enhance that change. Anti-biotics can be very harmful with long-term usage, and can lead to anti-biotic resistance later in life. Perhaps ask some betta breeders what they think once you get a clearer pic, so you can see if it is needed to continue medicating or not?
edit: Ooops, ye replied too quick for me

How about you try her alone in the 10 for a week and make sure she is in good health once she is off of meds. If her condition crashes and she starts showing signs of illness, try a new med. If she continues to be happy and healthy, go ahead and return her to her sisters. =)