Please explain why might two molars on the left, one upper and one lower gotten suddenly become very sensitive and painful?

Bruxism: Very likely since it is opposing teeth that are both suddenly sensitive and painful, you are grinding and clenching your teeth together at night while you sleep. Most people don't even know they do it. It is not caused by stress as many people think. The jaw muscles contract very forcibly in reaction to an imbalance when the teeth first come together while sleeping. See a TMJ trained dentist.

Tooth pain: So many unanswered questions. How long since you saw a dentist and had x-rays and an exam to check for cavities? Do you have sinus/allergy issues? ( increased pressure in the sinus can cause increased pressure on the upper tooth, causing more pressure on the lower. Are you grinding your teeth? Did you bite into something hard unexpectedly, thereby bruising the teeth?. Have you been chewing on ice?