Hello & welcome,
Symptoms you have mentioned here are typical of the infected tooth though it may have been treated by Root canal Therapy in the past.
If there is large infection noticed by your dentist
, then intake of Antibiotics & painkillers only will not be sufficient to resolve the problem.
I hope that your dentist had taken a x-ray of the same tooth which is must in your case to know the exact cause and extent of the infection.
Tooth which is treated by Root canal Therapy might get reinfected if there was some debris of necrotic pulp tissue remaining in the canal before obturation.
Another reason is the filling in the root canal being over extended or tooth might have got fractured.
Evaluation of adjacent teeth is very important to know the exact focus of the infection.
Final solution will be to repeat the root canal treatment
with proper sterilization
& accurately filling the canal with inert material to achieve intact periapical seal.
You might be feeling weak due to use of antibiotics which causes disturbance in gastric flora thereby reducing willingness to eat.
Eat high protein containing diet, drink plenty of water to avoid dehydration