Why ją and not jej?
Pain is something that in Polish usually kinda 'happens to you'. Your ear hurts to you. Your stomach hurts to you. I think a construction "jej ucho boli" (her ear hurts) is also possible but less common. I am a lot more likely to say "Boli mnie głowa" than "Moja głowa boli".
Ah, ok, thanks. I don't think we've seen much/any "ją" in the course so far, but that makes sense. Spanish is kind of the same way - me duele la cabeza - (my) head hurts me. Though that might actually be dative(?) - I'm not great at mapping cases to less-declined languages.
jej ucho - her ear, she owns the ear.
Russian: U niejo bolit ucho.