problem with chicken

don't know if anyone will be able to help me with this but it's worth a try. as i said in another post i have 4 12 week old chickens i brought last sat and 1 19 week old chicken we got last sun. the 19 week old was a friends and she was being bullied by the other chickens so we thought we would try her with ours. they have all been fine together. the only problem we've had is one of them has diarrhea. i'm pretty sure it is the 19 week old. i have only seen her do this. she's been like this for about 4 days now. other than that her general health is ok and she's eating and drinking fine. sorry for going on a bit. any help would be appreciated. thanks in advance.

will try 2moro. as she had been with other chickens and they was all ok i thought it would be ok. i am new to this so i'm unsure about alot of things even though i thought i'd done loads of research before hand.