Once the beer has started to sour it will usually get worse with time. Is it over carbonated? Be careful, as this bacteria takes hold of the beer more you might get more carbonation and might have possible bottle bombs.
The priming sugar being added to the top of the beer would not lead to sourness, unless there was a problem with sanitization. It is recommended to add it to the bottom of the bottling bucket so it has a chance to mix in well with the beer and you do not get some carbonated beer and others that aren't.
Open another bottle and make sure it was not just that one bottle that soured. Put all the beers in the fridge as well to slow the process of souring and never use soap in the dish washer, it is very hard to take off. Just make sure after you drink a beer rinse it out well so that no grime will build up in the bottles. Then you put them in the dishwasher on the sanitizing cycle with no soap. No rinse sanitizer will not clean grim off bottles or soap. It is meant to sanitize not clean, in more concentrated solutions it can clean but it will then need to be rinsed as well.
On your next batch try cleaning the bottles with PBW or Oxy if they are dirty then sanitize with sanitizer or the dishwasher without soap. Give the beer some time see what happens, but for now bleach bomb all your equipment and get started on another batch.
Hope this helped.