From the picture I would say it is likely that it is a she. some females can have some nice coloring (although the males are typically much brighter colored.) I can't say for sure since I can't see the fins. Here are some pictures that show what to look for
The top one is a female. Notice her slightly bigger size and the fins along her belly. Here is a female
The bottom is a male. Notice the different fins along his bottom along with his gonopodium. which look like this (the arrow points to the gonopodium)
Now if it is a female, you might want to seperate her. simply because so many males will stress her out to the point that she may die. I would say (especially if you wish to save some fry) to move her now.
If it is a male, I'm not sure I can really help as to why he's doing this.
Well even some of the good pet stores I've been to have sometimes put female guppies with all the males on accident.