Increasing sensitivity in tooth...
Based on you photo and your description, I would recommend that you see a cosmetic dentist or endodontist to have your tooth checked and re-treated. You may very well need a second root canal on that tooth and a replacement crown. If you had a root canal and crown placed 6 years ago at y office, I would expect it to last you 10-20 years, at least, but depending on your oral hygiene and frequency of dental check-ups, it's very possible for your teeth to need further treatment after only 6 years. I would recommend having the tooth examined by your dentist, as soon as you can, so that your pain can end and any infection can be treated before it destroys your entire tooth. Good luck and I hope this helps. Follow me if you have more questions.
Sarah Thompson, DMD
With the picture we cant see if you had a root canal therapy good or badly done. That only can be determined with a X-ray.
the only thing that can be seen with the picture its that your crown its not properly adapted due to a bad crown preparation.