My tooth hurts when im upside down or bend down. what can be causing the pain? i had a root canal done on same tooth about 3 years ago.

Sinusitis: Yes, those are classic symptoms of a sinus infection. However if that tooth is a lower tooth it may be getting reinfected. Either way you should see a dentist or doctor to confirm or rule out a sinus infection.

Get the tooth checkd: The concerning part here is the past history of a root canal. Many upper molar teeth commonly have complexities that contribute to a later infection. This infection can often spread to the sinus, so the 2 could be linked. I would not take an antibiotic without first being evaluated. An endodontist (preferrably one with a ct scanner) and that uses a micrscope would be best able to assess you.