Grey Spot on back of Incisor?!

I have just noticed a grey mark on the back of one of my upper incisors near the bottom edge of the tooth. I have been to the dentist 3 times in the last three months as I currently have an anxiety disorder and worry about my teeth. Each time I've been the dentist has taken and x-ray and there have been no signs of decay. Last time I went was a month ago and still had no problems after noticing other brown marks.

I clean my teeth thoroughly every day, brush twice a day for 2 minutes, floss and use mouthwash. This mark most likely has been there a while but I'm still worried, Could tooth decay develop that quick after an examination? Bearing in mind I haven't had decay in 6/7 years and just this past few months I have improved my oral hygiene considerably.

Could dental caries form in such a short space of time?

I thought it was a brown spot, but with a closer a look it seems to be grey....