Thanks for putting up your question on Just Answer.
Without looking at the X rays of those two teeth (to see how the root canals were done), we can just speculate as to the cause of the pain. Root canals are blind procedures, and therefore carry a success rate of about 90-95%. What this means is, that in every 5-10 out of 100 cases, the root canal has to be done again.
The main reason for this is that some amount of pulp tissue that was supposed to be removed in the first attempt, gets left behind. This happens because teeth have a lot of variation in their anatomy, all of which cannot be seen on X rays.
The reasons for your pain could be :
1. Inadequate instrumentation (some amount of pulp tissue is still left , which can be the source of pain)
2. An entire root canal missed (not picked up on an X ray and hence not cleaned)
3. A broken instrument being left inside the root canal which forms a nidus for infection leading to failure of the root canal.
4. The root canal is not filled entirely, which leads to an incomplete seal at the apex of the root, from which leakage of microbial products occurs and thus infection.
Discuss all these possibilities with your dentist. Get X rays done.
For now, take anti inflammatory medication. Switch between taking 400 milligrams of ibuprofen (e.g., 2 tablets of Advil), and 1000 milligrams of acetaminophen (e.g., 2 tablets of extra-strength Tylenol) every 3 hours. Be careful not to use these medications if you have had previous adverse reactions to either, or if you have had gastritis or peptic ulcer disease in the past, or if you have liver disease.
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Good Luck !