Why does my dental crown hurt nearly 3 years later? Sensitive to pressure and temperature changes.

I had a crown placed on on of my right molars around 3 years ago. It was very irritated and sensitive most of this time. My dentist has always told me my sensitivity was nothing to cause concern. I have mostly stopped chewing tougher foods on that side. Recently, I had been having less sensitivity, so I have been chewing on that side again. Now, I am having more pain and a constant ache since yesterday. Is the chewing to blame? What causes this? I want to be able to use that side of my mouth.