My top right molar tooth began hurting today out of no where. i was fine all day, until i pressed my teeth down. only hurts when i chew. what is it?

Clinical Evaluation: And x-ray is needed to come up with a definitive diagnosis. A cracked tooth syndrome can cause pain when pressure is placed on the tooth. It is very common on cases where there's a severe interference on teeth of somebody who has bruxism (grinding) habit.

See a dentist: Tooth could be fractured or infected. You need to see a dentist as soon as possible to determine what is causing the pain and what needs to be done about it.

To be determined: It could be one of many things- a broken filling, sore tooth from bruxism, a fracture in the tooth, an infection, a cavity, etc. Your dentist can determine what is wrong and what needs to be done to correct the situation. Don't wait, as this is a sign and the longer you wait, the more the chance of bigger problems to come.