In chapter 4 of The Lightning Thief, Percy faces physical challenges on top of dealing with an onset of emotional turmoil. He now knows that the people around him have been lying to him and there are monsters trying to end his life. He has a lot of questions but the immediate danger of the Minotaur makes answering them now impossible. All the group can really do is run, because this is one of the tougher monsters that could have come after them.
While Percy is ready to run at first, Grover and his mother being in danger quickly changes his flight response to a fight response. It is ultimately his mother's heroic sacrifice that enrages Percy to the point of being able to kill the Minotaur. Granted, it was ultimately as much luck as planning that saved him.
Percy gets injured during this fight, and as soon as it's over, he breaks emotionally. His mother is gone. All he can do at this point is cry and drag Grover to safety, which he barely manages to do before his head injury catches up with him and he passes out.