I, by no means, am a dentist, but if it really worries you, get a second opinion.
It could be that there is nothing wrong with your teeth and something else is causing the pain, like sinuses, or a nerve. My mom had a molar pulled years ago, and the top molar that is directly above it hurts her from time to time. The dentist says the tooth is fine.
It's my understanding that usually an infection shows up on the x-rays. But don't take my word for it. Like I said, if it really bothers you that the original dentist might have missed something, get a secong opinon.
Hope that helps somewhat