I am unable to get a doctors appointment as I have recently moved to cardiff.
Before coming I went to the dentist and had a deep filling.
As the dentist was doing the filling I was in so much pain that they had to use extra anesthetic.
It's been a few weeks now and suddenly, when I eat hot food my tooth with the filling starts having a sharp throbbing pain for about 5 seconds and then it goes.
What do you think this could be?
It sounds like the filling was close to the nerve, and those can be sensitive to heat and cold for a while, especially if it was a silver filling rather than a white filling. I had a deep silver filling, and it hurt if cold touched it for a few months. Then the sensitivity went away.
For now, I wouldn't worry too much. If it seems to be getting worse, like hurting with all pressure, try to at least call the dentist who did it.
the need for extra anesth is due to decay being close to the pulp. the dentist probably then went ahead and put in a sedative filling (a material that "calms" the nerve by buffering it so to speak from the final filling material that you can see). sometimes the nerve will build repairative dentin (although not terribly quickly) and the pain will subside with time. if this is the case i am sure you are naturally going to baby it on that side anyway to see how things go.
it's also ossible the decay had been just too deep and the dentist was being conservative, to give the tooth a chance rather than jumping straight into a root canal treatment.
if the time it takes to calm down after heat insult increases (that is, throbs for longer than the 5 seconds you have now) it means the pulp is entering into an irreversible state and it will let you know in no uncertain terms. at that point you will need a root canal, assuming the tooth structure itself is in generally decent shape.
5 seconds time for pain to go away is borderline and a longer than one would like. usually pain from heat or cold should go away (completely) in 2 seconds. but give it a chance. good luck!
Extreme pain in a filing when eating hot food 
