Black dots on teeth after filling

About 2-3 months ago I have had a couple teeth filled with the white filling because i had a couple small cavities.  
I have been brushing my teeth at least twice a day (morning and night) and flossing at night.  That is my regular routine.  Last night when I was flossing my teeth, I noticed a tiny black dot on one of the teeth I had filled.  It is pretty much touching the filling.  How could this be a cavity already?  So as I looked at the other teeth I had filled, the same thing.  A tiny black dot right almost touching the filling.  None of the teeth seem sensitive at all.  

It upsets me that this is only on the teeth that I just recently had filled.  No others.  Did they maybe not clean out all of the cavity before they filled it and now it is just getting worse?  Do I have to get the re-filled?  

[scottma]

Going back to your dentist to have a check up isadvised.