I also have a female who shows horizontal stripes all the time. She is happy as a clam and the dominant female in her group of sisters. She eats, she plays. Unfortunately, she is also the ugliest of the group, but I love her.
IDK why she sports those stripes. She has had them since babyhood, and they never completely go away, although they do become more pronounced if she is scared or stressed. Her main color is drab black with multicolored red/blue fins.
I think it is her coloring. She is also a true gaint (seriously she's got to be pushing 3.5 - 4 inches and she's stocky too). She's just a stripey ugly happy girl.
I would say watch your girl to see how she does for a while. It is commonly a sign of stress, but your may be an exception like mine.