Can an old (over 20 years old) root canal be infected even though nothing shows up in an X-ray?

I have 3 root canals on the bottom left side. They are all at least 20 years old. For the past 5 or 6 years I have problems. A few months ago the old crowns were taken off. Underneath there was a lot of decay. The last back two teeth were black. My dentist drilled them down & put posts in them to support them. He then put new crowns on them. The crowns are fine & my bite is fine but I still feel pressure & the middle tooth is sensitive. Could I have an infection & what should I do?