why Katniss didn't eat nightlock berries from the forest?

She remenbered what her father had said about the berries as well as she knew Foxface died from consuming them. 

The girl from District 5 ate Nightlock which can kill you within a minute and she ate and then she died. Peeta actually left them out. As a result, she collapsed on the ground and died, the cannon signaling her death, and the mockingjay giving a high pitched warning cry, letting Katniss know that the hovercraft was going to retrieve the dead tribute girl.

Foxface placed 4th in the Games. In the movie, Foxface died before Thresh did,

Foxface after eating the nightlock berries.

making Foxface come in 5th. Katniss and Peeta find her body holding nightlockberries. She died after stealing and eating the berries that Peeta had collected. Katniss believed that if Peeta had known about the poison and was attempting to trick Foxface, she would have known not to eat them and could have won. However it is thought that she knew about the nightlock berries being poisonous and committed suicide, due to the scene in the training centre where she matches up and tests her knowledge of various plants and berries. When her body was then taken away by a hovercraft, Katniss mentioned that her body was emaciated, so she would've died sooner or later from her lack of food supplies.