Yes it could be a failed root canal treatment
, may be some periapical infection left behind before completing root canal treatment,
sometime in grossly decayed teeh periapical granuloma
become which is difficult to treat only with RCT, and if you are feeling metallic taste
only on that side there may be some sinus
formation, from where pus is draining and causing the symptoms.
i will advise you to visit your dental surgeon and go for an check X-ray for proper treatment of these teeth.