Female mice fighting.....WTF??

[nickykci]

ok, i'm really confused here, everywhere i look on the internet, it sais that Females wont fight, YET mine do!!
i introduced a new female to a breeding pair... everything was ok for the 1st couple of days, the male did his job, then the older female had a litter...and the younger female seems to be attacking the new mother... i seperated them for the time being, but i'd like to add her back to the colony, and might add one more later on also...

i am really confused here, any help would help lol and also if anyone can clue me in on how to "sex" young mice...i'm having problems doing that, and i want to learn befor the babies start breeding themselfs lol