Fox, I had the very same thing a couple of years ago - Dentist decided to replace a back tooth filling -it had never bothered me. Once the injections wore off, it started to ache. Went back to Dentist 3 times, she replaced filling, and put a dressing under the filling. It was xrayed, but nothing could be found as to why it was hurting me.
I was going to have a root filling, as the Dentist thought this would be the answer, but the price of £900 put me off (private). Anyway, I didn't go back, and even though it took a while with painkillers, it did settle down. It is still sensitive if, I say, touch the filling with a spoon when eating. Metal against metal.
Hope yours settle.