It sounds like an periodontal abscess to me, abscess in the gum.. it is a chronic infection which becomes acute which swells up with pus and blood.

Cause of these abscess is due to tartar that may be well hidden under the gums.. usually you'll need an x-ray to be able to appreciate it.. a periodontal problem will not have any signs of decay or tooth problem. and mild discomfort only.

Please go ahead with another dentist for a second opinion. If it really is a periodontal abscess.. you will require a root planing or a deep scaling.

Here a picture for your reference,

From what you're telling me, it sounds alot like a gum problem. It doesnt not happen inside the tooth but on the outside which affects the supporting structure of the tooth namely the gums and the bone that are holding it.

Your dentist did a root canal that removes the pulp of the tooth which will render the tooth pain free cause it does not have anymore nerve endings. It is normal that there is no more pain on the tooth and allow you to bite.

He has cleaned the inside thoroughly but maybe the outside of the tooth hasnt been cleaned yet.

Its better to get a second opinion somewhere in this case. It hard to diagnose without an x-ray for your case. And your dentist telling you that it cant be cleaned thoroughly is kind absurd.. No matter how the tooth anatomy is, we are supposed to clean it out thoroughly when we do a root canal, and with you having gum issue.. its almost impossible to for him to not notice.. except he chose to ignore it..

unfortunately.. a gum treatment takes a longer time and need constant monitor and follow to ensure it heals and gets better.. charges arent high as compared to a root canal and a crown.. so yeah.. if you problem still persist.. i would suggest to see another dentist.