I have a porcelain crowned front tooth that is suddenlty giving me trouble. It is so painful that it is sensitive to the touch. I brush my teeth 3 to 4 times per day, so I know that it can't be a hygiene problem... Why am I experiencing this pain and sensitivity?

I think that is something that you need to go back to your dentist for...

Just because you brush a lot doesn't mean that your teeth are completely healthy, do you floss regularly?

The nerve of the tooth might be dying or if it's already had a root canal it may be re-infected. The other possibility is that you are clenching or grinding on it and it might need to have an adjustment.

Either way, make an appointment with your dentist today. He'll take an x-ray of it and should know then what is happening.