I Had Maryland Bridges To Replace Two Missing Lateral Incisors.

I had maryland bridges to replace two missing lateral incisors. Recently I have been feeling sensitivity and pressure on my left central tooth. There is a weird feeling on that tooth, so I took pictures of the backside. There seems to be two dark spot areas on the upper backside of the tooth. Can you please tell me what this is? If you need more information, please let me know. Thank you Here are the pictures: http://tinypic.com/r/33p32pd/9
http://tinypic.com/r/fjnhnt/9