Perhaps he was near a Dementor. Dementors are known for sucking out the happiness and warmth from their surroundings. If Harry was in close proximity to one, it would make him feel extremely cold. This is similar to what happened in the books and movies when Dementors were around.
It could be due to a spell. There are many dark spells in the wizarding world that might cause a person to feel cold. For example, some curses can drain the warmth from a body. If Harry was targeted by such a spell, he would be cold. Also, it might be related to his emotional state. If he was feeling lonely or scared, it could make him feel cold inside, just like when people are in a bad mood in real life, they might feel colder than usual.
One reason could be that Harry wants to preserve the integrity of his real story. Fanfiction often takes the original story in different and sometimes wild directions. He might worry that it could distort the truth about his adventures, his friends, and the sacrifices made. For example, some fanfictions might create romantic relationships that didn't exist or change the motives of characters, which could mislead those who are new to the real story of Hogwarts and his battles against Voldemort.
In fanfiction, Harry Potter might be angry if his friends are in danger and no one seems to be taking it seriously. He's very protective of his friends, like Ron and Hermione. So if some authority figures in the story are being dismissive of the threats his friends face, that would surely make him angry.
One reason could be a reaction to the Ministry of Magic's mismanagement. In the original story, the Ministry often made mistakes and had corrupt elements. In fanfiction, this could lead to Harry becoming an anarchist as a way to oppose such a flawed system.
Perhaps he has trust issues. He might have been betrayed in the past, so now he keeps everyone at a distance to avoid getting hurt again.
It could also be about a new character in a fan - created story within the Potter world. This character might possess a power related to cold fury, like being able to freeze things when enraged. And the fanfiction could explore how this power affects the character's relationships with others in Hogwarts or the wizarding world at large.
Another reason might be related to his family. In some fanfictions, his parents' story or his relationship with his family members could be distorted. Harry has a strong connection to his family, especially his parents, and any wrong portrayal of that could make him pissed off.
One reason could be the allure of power. Grindelwald was extremely powerful, and Harry, who has always been in the midst of power struggles, might be fascinated by that level of magical ability. Another reason could be a sense of curiosity. Harry has a thirst for knowledge, and Grindelwald's unique ideology about the wizarding world might intrigue him.
One possible reason could be that he's worried about Ginny's safety during the pregnancy. He might be overprotective and his concern comes out as anger.
It could be a case of misinformation. Someone in the story might have spread false rumors about Harry being the father for their own motives. Perhaps they wanted to cause chaos or gain something from the situation. So, the whole idea of Harry being the father is just a false story that gets spread around the wizarding community.
The author might describe his shivering. For example, 'Harry Potter was cold, his body shaking uncontrollably as he stood there.'