Olá, @gmliii. Common usage has created certain trends for some body parts. We might say dor de cabeça (headache), dor de garganta (sore throat) or dor de barriga (stomach ache), but otherwise, I’d say we generally use the default “dor(es) no …/ dor(es) na …”, which could also be used with the aforementioned body parts if we wanted to. Most of the time, there’s no practical difference between dor and dores when we use these two words. If in doubt, you can just use one and you’ll be fine