See there can be two things one which almost all of my fellow dentists have already suggested that the decay might be too deep and need RCT intervention but secondly the filling can be faulty also which might happen due to breech in isolation that means your tooth might have got wet during the fill leading to its failure..
Next Steps
Don't worry go and get a X RAY done, either post it here or show it to the nearby dentist and take appropriate action accordingly i.e. Either a refilling or a RCT
Health Tips
Meanwhile if it's just a sensitivity related issue try Colgate's PAIN OUT on that tooth