Hi! The brown spot you're seeing on your lower tooth, along with the sensitivity, could be an early cavity (tooth decay) or a surface stain. Since you're experiencing sensitivity, it's more likely that the enamel is slightly compromised, which needs attention.
Next Steps
It’s good that there’s no pain yet, but early treatment can help prevent it from worsening. I recommend visiting a dentist soon for a proper clinical examination and possibly an X-ray. If it's a cavity, a simple filling might be enough. If it’s just a stain or mild enamel wear, your dentist may suggest fluoride treatment or preventive care.
Health Tips
Use a toothpaste for sensitive teeth.
Avoid very hot, cold, or sugary foods.
Don’t try to scrape the spot or self-treat it.